#help-10

1 messages · Page 292 of 1

grave crypt
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if we looked at it from the right angle

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op / hyp

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wait

azure anchor
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sin is the y axis of the unit circle

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radius of unit circle is 1

grave crypt
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i was never showed a unit circle sorry lol

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but continue

grave crypt
azure anchor
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yes

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1/-1 =

grave crypt
#

oh alr

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so applying that for e and f of 4

azure anchor
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positive angle only

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for what positive angle is cot = -1

grave crypt
#

do you mean like cast rule

azure anchor
#

?

grave crypt
azure anchor
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cot is identity for what

grave crypt
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1/tan

azure anchor
#

and tan is the identity for what

grave crypt
#

sin x cos?

azure anchor
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tan = sin/cos

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1/tan = cos/sin

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so there's an angle where cos/sin = -1

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when does cos/sin = +-1?

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what special triangles have both adjacent and opposite sides the same?

grave crypt
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45-45-90

azure anchor
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tan = opposite/adjacent

grave crypt
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yeah

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so

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1/1

azure anchor
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but we need negatives

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so what quadrants do you think that would happen

grave crypt
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2 and 4

azure anchor
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what angles would give us 45 45 90 triangles in those quadrants?

grave crypt
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180-45

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so

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135

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and

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360-45

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315

azure anchor
#

looks like you got it

grave crypt
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okay last one

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please

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and thank you

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sin = 1

azure anchor
#

sohcahtoa
sin = opposite/hypotenuse

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what type of triangle you think we need for sin to be 1?

grave crypt
#

dont none of them give that

azure anchor
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opposite/hypotenuse = 1 means opposite = hypotenuse. so we can still use a 45 45 90.

grave crypt
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waitcant i just look at a singe function

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sin at 90 is - 1

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=1

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sine

azure anchor
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sin 90 = 1, right. like i said, sin is the y-axis of the unit circle/triangle.

grave crypt
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hm okay

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but i still wanna hear your way

grave crypt
azure anchor
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imagine theta is our angle, ad we know that in this case sin theta = 1

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crap hold on

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its been so long since i had to use triangles

grave crypt
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is it going to be easier then

azure anchor
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i actually cant really think of a way to visualize sin theta = 1 with triangles and the opp/hyp definition

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sorry

grave crypt
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that’s okay

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i’m going to skip this and move to radians

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is it easier for you to explain with radians then?

azure anchor
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if i can use the unit circle, then yes.

grave crypt
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are you meant to memorize the unit circle

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this is confusing

tardy epoch
grave crypt
#

could you try explaining the values

tardy epoch
obtuse pebbleBOT
#

@grave crypt Has your question been resolved?

obtuse pebbleBOT
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silent otter
#

i dont rly understand what i have to prove here apart from commutativity of addition

silent otter
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heres def 5.18

silent otter
obtuse pebbleBOT
#

@silent otter Has your question been resolved?

obtuse pebbleBOT
#

@silent otter Has your question been resolved?

obtuse pebbleBOT
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honest flicker
obtuse pebbleBOT
honest flicker
#

I have this so far idk what to do

final hornet
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so thats the volume of the sphere

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it has a density of 0.9 kg/m^3

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multiply that by the volume and you should get the mass

honest flicker
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Idk😭

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I don't think that's the weight of the air

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Oh wait kilograms

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Idk how much that is

final hornet
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idk

honest flicker
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Is that the answer

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@final hornet

final hornet
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in theory yes

final hornet
honest flicker
#

I'm scared

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Omg I got it right

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Ty

#

.close

obtuse pebbleBOT
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thin pendant
#

how do I solve this? (sorry for rotated photo)

narrow vault
#

,rccw

warm shaleBOT
lethal breach
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first you'll need to find the angle AOB (considering O is the center of the circle)
well, technically thats the answer

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in radians

thin pendant
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ok thank you

#

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obtuse pebbleBOT
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compact gate
#

.close

obtuse pebbleBOT
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dawn vessel
#

How to do 22b) and 22c) ??

obtuse pebbleBOT
dawn vessel
#

Also 23b)iii)

lethal breach
dawn vessel
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BEC = CFB

lethal breach
lethal breach
dawn vessel
lethal breach
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they haven't provided any info

dawn vessel
dawn vessel
dawn vessel
lethal breach
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who told you that?

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there isnt any info given. you cant just assume

dawn vessel
lethal breach
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trust

dawn vessel
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do you see the 2 triangles in EADB

lethal breach
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i'll show you one example. one sec

dawn vessel
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AEB = ADB

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they share 2 same points

dawn vessel
lethal breach
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@dawn vessel

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they have the same side AB

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or am i trippin

dawn vessel
lethal breach
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i am

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yes

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i am trippin

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fuck

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youre right

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okay i get it

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im sorry

dawn vessel
dawn vessel
lethal breach
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nono. i get it

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the last time i did circles was 3yrs ago

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its hard to remember

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now that i do

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yes

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mb

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im sorry

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so yes

dawn vessel
dawn vessel
lethal breach
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so. angle EAB is 90
cuz in triangle EAB, you know E and B. so A is 180 - those 2 angles = 90

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since EAB is 90, ECB is also 90

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thats c) done

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@dawn vessel

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now for b)
you can use the quadrilateral ABCE. you know E, A and C. you can find B

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does this help?

dawn vessel
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one sec lemme see

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you are correct for c)

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so b) will be 138 right? @lethal breach

lethal breach
dawn vessel
#

ight thx

dawn vessel
#

.close

obtuse pebbleBOT
#
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uneven horizon
#

why is r=4?

obtuse pebbleBOT
fierce lagoon
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Because it's a circular arc centered at the origin. It's pretty clear that r goes from 2 to 4

uneven horizon
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actually im not very clear on polar coordinates

fierce lagoon
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It's legit on the graph

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Basically a circle.

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x^2 + y^2 = 4

uneven horizon
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so is r supposed to be a radius

fierce lagoon
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Yeah

uneven horizon
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oh

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🗿

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well that was very stupid

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then why isnt the lower limit 2 if we apply the same logic

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my prof did this

fierce lagoon
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Oh they're tryna find the pink line

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The equation of the pink line

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x = 2

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rcos(θ) = 2

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Since x = rcos(θ)

uneven horizon
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oh is it cause now we do polar coords so we cant take x

fierce lagoon
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Mmhm

uneven horizon
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but if thats the case

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then why wouldnt the upper limit be rcos(θ) = 4

fierce lagoon
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There's no vertical line at x = 4

uneven horizon
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then 4secθ

fierce lagoon
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The region is bounded by a circular arc (the red curve) on the right sode

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It's not a vertical line

uneven horizon
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so i take the arc instead?

fierce lagoon
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Well yeah

uneven horizon
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but isnt 4 the radius

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its not the arc right

fierce lagoon
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r = 4 on polar coordinates is the arc

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Well

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The full circle

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But the arc in this context as a bounding curve

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Here hold on

uneven horizon
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okie

fierce lagoon
uneven horizon
#

i think i kinda get it now

fierce lagoon
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Yeah I made the full bound. Desmos is being a bitch and won't let me graph the stupid horizontal line but there you can see the region

uneven horizon
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yes

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and what about finding the theta limits?

fierce lagoon
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Hm

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Gonna dig deep into my brain for this because I kinda willfully forgot how to do some of this lmao after a month of pure mental torture

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θ obv starts at 0

uneven horizon
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0 and something pi

fierce lagoon
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Well, in most nice cases

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Until you get into cones and shit, no

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They can be ass

uneven horizon
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like triple?

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integrals

fierce lagoon
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Yeah

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Well actually

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No you can just use pythag

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The upper bound is arccos(2/4)

uneven horizon
fierce lagoon
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The red curve is a circular arc of radius 4 to any distance from the origin to any point on the red curve has to be 4

uneven horizon
#

oh rightttt

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okay i got it now

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thank you so much!

#

.close

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#
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undone plume
#

HOW DO YOU FIND THETHA

obtuse pebbleBOT
undone plume
#

😢💔

pulsar quarry
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when is value of sin equal to root 3/2

undone plume
#

huh

shut lagoon
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They are just known identities. Look at this .

undone plume
#

how in the world am i supposed to memorize that for a quiz

timid silo
#

so with this it is simple trigonometry and equations

warm shaleBOT
#

Mr. Macro

timid silo
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we can use this formula to solve for theta by rearranging the formula, but be for so we must substitute the given values

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$Tan(\theta)=\frac{(\frac{(\frac{\sqrt{3}}{2})}{(\frac{1}{2})}$

warm shaleBOT
#

Mr. Macro
Compile Error! Click the errors reaction for more information.
(You may edit your message to recompile.)

timid silo
#

oo

undone plume
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how do you know you need to use tan

timid silo
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dam i had the solution but it was to much for the bot

undone plume
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LOL

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no but why tan

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why not sin or cos

ember frost
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$\tan{\theta}=\frac{\frac{\sqrt3}2}{\frac12}$

undone plume
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silly bot

timid silo
undone plume
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yes

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i know how to find them

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but i dont know how to find the missing angle

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using the sin cos tan thing

timid silo
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ok wait i have to fix the bot

undone plume
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okay

timid silo
#

$Tan(\theta)=\frac{(\frac{(\frac{\sqrt{3}}{2})}{(\frac{1}{2})}}$

warm shaleBOT
#

Mr. Macro

$Tan(\theta)=\frac{(\frac{(\frac{\sqrt{3}}{2})}{(\frac{1}{2})}}$
```Compilation error:```! Missing } inserted.
<inserted text> 
                }
l.52 ...rac{(\frac{\sqrt{3}}{2})}{(\frac{1}{2})}}$
                                                  
I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing `2' now; then
my insertion and my current dilemma will both disappear.```
timid silo
#

GRRRRR

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why

lilac pebble
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u learn the unit circle before sohcahtoa??

undone plume
#

yes 😢

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my teacher is a bit silly

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he thinks we remember everything from last year

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but i forgot

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maybe im the silly one

lilac pebble
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T=o/

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t=o/a

warm shaleBOT
#

FungusDesu

ember frost
#

there

timid silo
#

yessssss

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WOOOAA

undone plume
#

but why tan

timid silo
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we get the value oppisit toit

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and the value adjacent

undone plume
#

what about sin

timid silo
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given tan = opposite \ adjacent

undone plume
#

that has oppisite

timid silo
#

you will still get the same

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since all sides are given

undone plume
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how does it know which angle

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this is so hard 😢

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why 😢

timid silo
#

$Tan(\theta)=\frac{(\frac{\sqrt{3}}{2})}{(\frac{1}{2})}$

warm shaleBOT
#

Mr. Macro

timid silo
#

you could use any

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ok now the next step

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we want theta alone so we must remove tan

undone plume
#

OHHH

timid silo
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when we divide tan we must inverse it

undone plume
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i thought that was the only step

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LOL

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so divide the other side with tan?

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how do you do sin inverse

timid silo
#

$\theta=Tan^{-1} (\frac{(\frac{\sqrt{3}}{2})}{(\frac{1}{2})})$

warm shaleBOT
#

Mr. Macro

undone plume
#

ooooo

timid silo
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well i since

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all the sides are given

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we could use cos, sin or tan

undone plume
#

wait let me try for cos

timid silo
#

ok

undone plume
#

yay it works

timid silo
#

😁

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i got the same answer with tan and sin

undone plume
#

if one of the sides was negitive would i have to change the side to a positive

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or does that make no sense

timid silo
#

ohhh like if the measurement of one side is forsay -5

undone plume
#

yes

timid silo
#

yes you would have to use -5

undone plume
#

okay

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also what is this asking

timid silo
#

unit circle

undone plume
#

whats a terminal arm

timid silo
undone plume
#

😟

timid silo
undone plume
#

do i just do the oppisite

timid silo
undone plume
#

yes

timid silo
#

and the rule how opposite angles are equal

undone plume
#

what

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how is it equal

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equal to what

timid silo
#

no no not opp

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opposite angles on a parallel

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When any two straight lines intersect each other, there are different pairs of angles that are formed. The angles that are directly opposite to each other

undone plume
timid silo
undone plume
#

ohhhh

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oh yeah

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oops

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so 180 + 60

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OH

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I GET IT

timid silo
undone plume
#

ITS 240

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😍

timid silo
#

wait which are you solving

undone plume
#

d

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wait a sec

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240° is not a coordinate

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💔

timid silo
#

💀 ive never learnt unit circles

undone plume
#

me too

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i was sick on that lesson

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wait wont it just be the same coordinates but oppisite signs

timid silo
#

ima quickly learn it and try to help

undone plume
#

this is the answer

timid silo
#

nice

undone plume
#

but how

timid silo
#

ohhhhh

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think of a number plane

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what is 4 - 10

undone plume
#

what is a number plane

timid silo
#

-4

undone plume
#

-6

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what

timid silo
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wait

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i mean

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what is the opp +4

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-4

undone plume
#

yes

timid silo
#

its flipped like a mirror

undone plume
#

i see

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so x is opposite sign

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yay

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🎉

timid silo
#

🎉

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wait wait

undone plume
#

wait is that wrong

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OH NO!

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wait no

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its right

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yay 🎉

timid silo
#

it depends on which you are looking for

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here is a diagram for all sides

undone plume
#

the question says pi - thetha

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180 - 60

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120

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so the one with the smiley face

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help

timid silo
#

Idk <@&286206848099549185>

undone plume
#

😢

obtuse pebbleBOT
#

@undone plume Has your question been resolved?

timid silo
#

Oh dear

obtuse pebbleBOT
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long bluff
#

hello

obtuse pebbleBOT
long bluff
#

i made a script in matlab for a predator-prey model

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i just need something explained to me

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so these are 3 graphs I made from 3 sets of inputs

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the first being 4000 prey, 80 predators

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second: 5000, 100

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third: 200, 50

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The second and the third graphs make sense, the first absolutely makes no sense to me

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why did that happen?

lone echo
#

without the script itself we cannot really know much

long bluff
#

sure 1 min

wooden cipher
#

Seems like a fixed point kinda thing

candid rock
#

The second graph is just a weird way to plot a limit cycle

wooden cipher
#

Im not the best at differential equation, but it has to do with that kinda stuff

candid rock
#

It makes sense if you think about it

lone echo
#

the cycle seems pretty weird actually. The populations should tend to the stability point, not oscillate so wildly

candid rock
#

Do you have some kind of time delay

long bluff
#

well its a very basic model with simple differential equations

lone echo
#

yeah but they dont look like it would be correct at the most basic level

long bluff
#

this is the model

lone echo
#

Originally, let's assume you have a (virtually unlimited) amount of prey and 1 predator

#

predator has free reign so it multiplies

long bluff
#

i built the script using eulers method

lone echo
#

until the prey is depleted enough, where you have k prey, n predators, and the n predators start competing

#

then the growth starts being slower, until it stops growing

long bluff
#

@lone echo that seems to happen clearly in images 2 and 3 but i dont understand how 1 happened

lone echo
#

at that point, you have a stable k2/n2 relation between prey and predators

long bluff
#

just from arbitrary values of 4000 and 80

lone echo
long bluff
#

in image 3 yes?

lone echo
#

your oscillations are constant tho

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oscillations should converge to 0 as the population gets closer to the stability point

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Something along these lines

long bluff
#

true, this model is intentionally unrealistic

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from what ive learned in class to make it realistic we add another factor to the diff eqs

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to achieve that image

lone echo
#

fair enough

long bluff
#

but for right now in this assignment I have to explain the failures of this model i showed you

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so far ive got what you said, but there is something else related to that first image

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I dont understand why it occured that way

lone echo
#

okay, your problem is with the graph that represents a straight line, correct?

long bluff
#

yeah

#

which is 4000 prey, 80 pred

#

if I change it to 4100, 80 or 4000, 70 it looks more normal with oscillations

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but for some reason at specifically 4000,80 it produces those straight lines

lone echo
#

can you represent, on the pop vs time graphs, the predator multiplied by 1000?

long bluff
#

like 4000, 8000?

lone echo
#

(or by whatever factor makes both pops be in the same order of magnitude)

long bluff
#

okay sure ill test that

lone echo
#

because on these kind of models, the important factor is not "how many" you have of each, the important factor is the proportion

#

okay, that was a misunderstandingç

#

i'm not asking you to introduce more predators

long bluff
#

i think that apparent rise in predators at the beginning is misleading btw, because the graph starts at t=1

lone echo
#

im asking you to represent, instead of the number of predators, 1000 times the predators that there are

long bluff
#

im confused

lone echo
#

let's say oyou have 4k prey and 40 predators

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represent the 4k prey, and tell it to represent 100 times whatever predators it has

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in other words, change the scale for the pred graph

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or represent them in different graphs

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so you only have prey vs time in one, and predator vs time in another one

obtuse pebbleBOT
#

@long bluff Has your question been resolved?

obtuse pebbleBOT
#
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sly matrix
#

can someone check this through? idk if this is the correct method

sly matrix
obtuse pebbleBOT
#

@sly matrix Has your question been resolved?

sly matrix
#

<@&286206848099549185>

obtuse pebbleBOT
#

@sly matrix Has your question been resolved?

obtuse pebbleBOT
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olive heath
#

Could someone please explain this to me?

obtuse pebbleBOT
drifting wraith
#

i don't understand what it's saying

#

it shouldn't be true though

#

the function that relates angle to x-coordinate is cos, not sin

#

maybe it starts at 12 o'clock

#

still doesn't work

#

it works if it starts at 6 o'clock

viral blade
#

Wait nvm

#

You're right

polar fossil
#

anticlockwise = positive Smile_02

viral blade
#

I read anti-clockwise and thought ah yes the negative direction

olive heath
#

um

#

so it's false?

drifting wraith
#

there's not enough information, it's mostly false

polar fossil
#

think about whether it's correct at ø = 0

olive heath
#

hm

past sand
#

"with respect to an angle ø"... they don't specify where the angle is

olive heath
#

hm

#

i think that's the angle

past sand
#

I suppose they want you to assume it's the angle between the origin-ball vector and the x axis, but it's a bit of a stretch honestly

olive heath
#

from starting point x = 0

viral blade
#

Presumably the standard position then

olive heath
#

but it's still going anti-clockwise

past sand
olive heath
#

yeah

#

but then it moves to the left

#

but that would mean negative x

#

but sine x goes positive first

#

is that good reasoning?

past sand
#

Not really

drifting wraith
#

it is

past sand
#

If the ball starts at (0,-3) (and ø starts at 0) then 3sin(ø) would describe its x coordinate

olive heath
#

yeah

past sand
drifting wraith
#

0,−3, bottom of the circle, 6:00

olive heath
#

oh, -3

#

right

#

but i doubt that's the case

#

do you know if you go anti-clockwise from the x axis (on the cartesian plane) to measure angle?

drifting wraith
#

the chance that it's true because it's like "i didn't say it doesn't start there" exists, but it's small enough to ignore

olive heath
#

hm

past sand
#

At t=0, (sin t, cos t) = (1, 0) so it is on the x axis, to the right

#

t is the angle here, just like ø in your problem, but your problem never specified what ø represents exactly

olive heath
#

oh

#

i was just

#

doing that lol

past sand
#

Yeah but this goes clockwise

#

Do -cos n to go anticlockwise

olive heath
#

ah

autumn adder
#

Well people doing Desmos and not telling me

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hushed seal
#

okay so im doing heat transfer and i seem to be getting the calculations wrong, asking chat gpt to give me questions to practice and when i do the calculations on my own, they seem to be different from the answers i ask it to give.

hushed seal
#

lemme show you

tardy epoch
#

No thanks

hushed seal
#

can i at least show you the question and the answer i got?

#

really, i just need someone to tell me if im getting these right or not

obtuse pebbleBOT
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@hushed seal Has your question been resolved?

neon vector
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You'll have more luck finding such tasks by searching on the internet rather than asking ChatGPT anything other than factual questions.

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@nimble gulch Has your question been resolved?

nimble gulch
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<@&286206848099549185>

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nimble gulch
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.close

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tawny kite
obtuse pebbleBOT
tawny kite
#

for the case yRx I do not understand how transitivity plays its role here?

#

I understand all the rough work

frosty spoke
#

I didn't read the whole thing, but I suspect that suppose you have a singular element and a maximal element of the set of size n

#

then if the singular element is "bigger" than the maximal element of the size-n set

#

then you need to show that the singular element is bigger than all of the elements of the size-n set

#

and you use transitivity to show this

tawny kite
#

say case yRx, the solution says that suppose z in B, x<z, so y < z, This is to show how if the maximal x is smaller than z then y is also smallet than z because y<x. But z can't be in B if x is maximal

#

but then if you are already claiming x to be maximal why do you have to say z not in B{x} by maximality at y?

frosty spoke
#

okay yeah this solution kinda makes it a little confusing because it proves it by contradiction

#

it supposes that there is a z in B that is bigger than x

#

and then uses transitivity to show that z is bigger than y, which contradicts the maximality of y (because y is alleged to be maximal in B and z is alleged to be in B)

#

I guess you have to do it this way, because it's a partial order

tawny kite
#

Ahh okok I dont want to close this channel cause I still wanna continue this question, but I have 2 and a half hours of lecture after

#

can I just keep this channel

frosty spoke
#

I think it'll autoclose after a while

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split pecan
#

I need immediate help please

obtuse pebbleBOT
split pecan
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<@&286206848099549185>

regal tundra
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@split pecan Has your question been resolved?

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crystal hull
#

Hi. I am trying to understand the Möbius inversion formula properly. I've digested the proof of the version, where the arithmetic function f and its summator F are being inverted via the Möbius function mu. However, it is not clear why this proof should hold for the case, when f and F are not arithmetic functions, but just mappings from natural numbers N to any abelian group G.

crystal hull
#

How can we be multiplying here, if it's an additive abelian group for example?

#

<@&286206848099549185>

kind hawk
#

you are multiplying by an integer

#

which is basically repeated addition

#

which is fine in an abelian group

crystal hull
#

Ok, i was just wondering if that's ok, i need to just write down the justification properly. Thank you. More in #advanced-number-theory

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timid silo
#

Solve for BD

$AC^2= \frac{BD^2+BD^2 \times Tan(\Theta)^2 \times Tan(\alpha)^2 }{ Tan(\theta)^2 \times Tan(\alpha)^2}$

warm shaleBOT
#

Mr. Macro

timid silo
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wait

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.close

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low ravine
#

help please

obtuse pebbleBOT
#

Please don't occupy multiple help channels.

low ravine
#

hello?

pulsar quarry
#

!1c

obtuse pebbleBOT
#

Please stick to your channel.

timid silo
#

hm

#

find the current

#

then subtract that thing from both the downstream and upstream time

obtuse pebbleBOT
#

@low ravine Has your question been resolved?

obtuse pebbleBOT
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@low ravine Has your question been resolved?

obtuse pebbleBOT
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@low ravine Has your question been resolved?

teal turret
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.close

obtuse pebbleBOT
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alpine oar
#

Hello, I am stuck trying to alter the form of the equation to the general form of linear differential equations of the first order and the first degree

obtuse pebbleBOT
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@alpine oar Has your question been resolved?

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.close

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mossy thorn
obtuse pebbleBOT
dark vector
#

have you drawn the figure yet?

obtuse pebbleBOT
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@mossy thorn Has your question been resolved?

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silk maple
#

is there a quicker way to solve this than partial integration?

forest sinew
#

power reduction?

#

a few times

silk maple
#

does that work?

#

i just simplified it to this

forest sinew
#

hm this may lead to tabular

#

but might be fast

silk maple
#

this integral is just 0

#

but it took me forever to figure it out

forest sinew
#

yea... write...

silk maple
#

ohh you can probably say that the integral from 0 to 2pi of any sin or cos to some uneven power is always 0

forest sinew
#

$\cos x \cos 2x = \cos(2x-x) - \sin2x \sin x$

warm shaleBOT
#

jan Niku

silk maple
#

ty anyway

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.close

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smoky dove
#

what is the asnwer to this question

obtuse pebbleBOT
smoky dove
obtuse pebbleBOT
smoky dove
#

@versed wolf

obtuse pebbleBOT
oak marlin
#

How did you get D

smoky dove
#

ok so if z is cis-2pi/3 then the conjugate must be cis2pi/3 so if we square z by 4 then we add cis-2pi/3 4 times to get 2pi/3

runic void
#

How do you represent z ?

smoky dove
runic void
#

I mean z= e^-2pi/3 right?

oak marlin
#

cis(-4*2pi/3) = cis(-8pi/3) = cis(-8pi/3 + 6pi/3) = cis(-2pi/3)

smoky dove
#

the modulus in this question is a bit iffy since it should not remain constant

runic void
#

$z* z’= |z|^2$

warm shaleBOT
runic void
#

since you are in unit circle

#

$z* z’= 1$

warm shaleBOT
runic void
#

or z’= 1/z

smoky dove
#

is it B then?

oak marlin
#

Yes

smoky dove
#

ah I see my mistake

#

I added the arguments inscorrectly

#

thank you guys 👍

runic void
#

This just means, the conjugate of your complex number is the multiplicative inverse

oak marlin
#

👍

runic void
#

This particularly forms a group under complex multiplication

smoky dove
#

.close

obtuse pebbleBOT
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low matrix
obtuse pebbleBOT
low matrix
#

i'm kinda confused by the leaking and pumping at the same time, help would be appreciated!

#

it's related rates btw

obtuse pebbleBOT
#

@low matrix Has your question been resolved?

mighty lava
low matrix
obtuse pebbleBOT
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@low matrix Has your question been resolved?

static sandal
#

if this hjelps

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forest sinew
obtuse pebbleBOT
forest sinew
#

just rewrite $\cos ^3 x + 1 = \cos x (1-\sin ^2 x) + 1$

warm shaleBOT
#

jan Niku

forest sinew
#

this leads to a very natural and easy u sub integral as the "hardest part"

#

$\int \cos x - \int \sin ^2 x \cos x + \int$

warm shaleBOT
#

jan Niku

forest sinew
#

clearly only $\int _0 ^{2\pi} \dd x$ is the only one with value over 0 to 2 pi

warm shaleBOT
#

jan Niku

forest sinew
#

just figured id say

#

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high vortex
#

Hello. I am having a world of problems trying to understand how to work out part b in this problem. I'm really not seeing the big picture. Any poiinters??

Laplace's equation in two dimensions is the PDE for $u(x, y)$
$$
\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}=0 .
$$

Such $u$ are "steady-state" (i.e., time-independent) solutions to the 2-dimensional heat equation. Consider this on $\left[0, L_1\right] \times\left[0, L_2\right]$, with vanishing Neumann boundary conditions on the left, right, and bottom sides:
$$
\frac{\partial u}{\partial x}(0, y)=0=\frac{\partial u}{\partial x}\left(L_1, y\right), \frac{\partial u}{\partial y}(x, 0)=0 .
$$

We'll analyze that boundary value problem (with vanishing Neumann conditions along 3 of the 4 sides).
(a) Show that the nonzero separated BVP solutions $X(x) Y(y)$ are the functions
$$
c_n \cos \left(\left(n \pi / L_1\right) x\right)\left(e^{\left(n \pi / L_1\right) y}+e^{-\left(n \pi / L_1\right) y}\right)
$$
for $n \geq 0$ with scalar $c_n \neq 0$ (don't overlook $n=0$, related to constant solutions to an ODE).
(b) Consider solutions $u(x, y)$ to the BVP subject to the further Dirichlet-style boundary condition $u\left(x, L_2\right)=f(x)$ along the top side, for a given function $f:\left[0, L_1\right] \rightarrow \mathbf{R}$ vanishing at the endpoints. Writing $u(x, y)$ as an infinite series
$$
\sum_{n=0}^{\infty} c_n \cos \left(\left(n \pi / L_1\right) x\right)\left(e^{\left(n \pi / L_1\right) y}+e^{-\left(n \pi / L_1\right) y}\right)
$$
in separated solutions, express the $c_n$ 's in terms of Fourier-type integrals involving $f(x)$.
Hint for (b): the $2 L_1$-periodic series $\sum_{k=0}^{\infty} a_k \cos \left(\left(k \pi / L_1\right) x\right)$ computing the even extension $f_{\text {even }}:\left[-L_1, L_1\right] \rightarrow \mathbf{R}$ of $f(x)$ has coefficients given in Proposition 21.1.8(i) with $L=2 L_1$.

warm shaleBOT
obtuse pebbleBOT
#

@high vortex Has your question been resolved?

silk maple
obtuse pebbleBOT
#

@high vortex Has your question been resolved?

tardy epoch
#

Probably should show the proposition mentioned

obtuse pebbleBOT
#

@high vortex Has your question been resolved?

high vortex
#

Proposition 21.1.8. Let $f(x)$ be an $L$-periodic function, and define $\omega=2 \pi / L$.
(i) For even $f(x)$, the associated Fourier series is $a_0+\sum_{k=1}^{\infty} a_k \cos (k w x)$ ("cosine series") with $a_0=\frac{1}{L / 2} \int_0^{L / 2} f(x) d x, \quad a_k=\frac{2}{L / 2} \int_0^{L / 2} f(x) \cos (k \omega x) d x$ for $k \geq 1$.
(ii) For odd $f(x)$, the associated Fourier series is $\sum_{k=1}^{\infty} b_k \sin (k \omega x)$ ("sine series") with
$$
b_k=\frac{2}{L / 2} \int_0^{L / 2} f(x) \sin (k \omega x) d x \text { for } k \geq 1 \text {. }
$$

warm shaleBOT
obtuse pebbleBOT
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pale drift
#

can a linearly dependent set of vectors be orthogonal?

sage geode
#

hmmCat Perhaps if there are two vectors and one of them is the zero vector

#

Can't see any other possibility

#

Or if it's just {0}

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slate saddle
#

How would I go about solving this?

obtuse pebbleBOT
slate saddle
#

Don't tell me the answer but what identities would I use?

#

okay

limber quartz
#

angle addition formula for cosine

slate saddle
#

hmm

#

Thank you I will try it out.

limber quartz
#

^

limber quartz
#

(eventually)

slate saddle
#

yeah

#

I am

limber quartz
#

because the angle addition identies follow a pattern

#

for cosine, it's cos(A +/- B) = cos(A)cos(B) -/+ sin(A)sin(B)

#

for sine, it's sin(A +/- B) = sin(A)cos(B) +/- cos(B)sin(A)

slate saddle
#

hmm

#

What about the product to sum identities?

#

Could that be used in this scenario?

limber quartz
#

you should use the angle addition formula for cosine.

#

get 1 by itself on RHS, factor out the 2 on the LHS.

slate saddle
#

okay I am trying it out

#

So

#

2cos(-3x)cos(2x) = 1 + 2sin(-2x)sin(2x) can be translated as cos(-3x +/- 2x) = cos(-3x)cos(2x) +/- sin(-3x)sin(2x)?

#

@limber quartz

#

thats for the LHS

limber quartz
#

you messed it up or typed it wrong not quite: you kept my notation for the signs when you should just stick to what you were given

#

rearrange what you were given.

#

get the 1 by itself.

#

on the left hand side, factor out the 2.

slate saddle
#

oh

limber quartz
#

then divide both sides by 2

#

the left hand side will be in the same form as the angle addition formula for cosine.

#

translate that

limber quartz
#

you are kind of right

#

but

#

pick which sign you have

#

you have subtraction on the right, which is associated with addition on the left (that's why it's the angle ADDITION formula for cosine)

slate saddle
#

okay

#

This is what i have

limber quartz
#

don't worry about my formulas

slate saddle
#

:

limber quartz
#

yet

slate saddle
#

2cos(-3x)cos(2x) = 1 + 2sin(-2x)sin(2x) can be translated as cos(-3x)cos(2x) - sin(-3x)sin(2x) = 1/2?

limber quartz
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yes

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good job

slate saddle
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yes finally

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then

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Thank god

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I finally see it

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Imma solve it then send the answer (if you have time to review it)

limber quartz
slate saddle
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not yet

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am doing it now

limber quartz
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use the formula sheet

slate saddle
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yessir

limber quartz
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$$\cos{(A \pm B)} = \cos{(A)}\cos{(B)} \mp \sin{(A)}\sin{(B)}$$

warm shaleBOT
#

Disorganized

slate saddle
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cos(-3x + 2x) = cos(-3x)cos(2x) - sin(-3x)sin(2x)

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Is what I have so far.

limber quartz
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good

slate saddle
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What happens to the 1/2?

limber quartz
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tell me.

slate saddle
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does that appear afterward

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?

limber quartz
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it's happening right now

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!

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😛

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transitive property

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"if x = y and y = z, then x = z"

slate saddle
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got it

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so

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cos(-3x + 2x) = 1/2

limber quartz
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yes

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keep going

slate saddle
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cos(-3x + 2x) has the result of:

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cos(-x)?

limber quartz
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yeah

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what else do we know about the cosine function that will simplify that even more

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"cosine is an (blank) function"

slate saddle
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umm

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let me think

limber quartz
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what does this mean:
f(-x) = f(x)

slate saddle
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odd

limber quartz
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no

slate saddle
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an odd function

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whoops

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oh

limber quartz
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odd is
f(-x) = -f(x)

slate saddle
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even then

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since

limber quartz
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yeah

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we can see it graphically as y-symmetry

slate saddle
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the "-" has to equal "+" for it to be even

limber quartz
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which cosine has

slate saddle
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hmm

limber quartz
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phrasing is a little imprecise

slate saddle
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rather

limber quartz
#

cosine is an even function because you can fold the graph over the y-axis onto itself.

slate saddle
#

yep

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okay

limber quartz
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algebraically (or graphically, if you plot it), for any input x, cos(-x) = cos(x)

slate saddle
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so the final answer (without finding the solutions in the interval) is cos(x)?

limber quartz
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no

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that's not the final answer

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this is the next logical step

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use the fact that cosine is an even function with transitive property again

slate saddle
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x = (2k + 1) pi/2?

limber quartz
#

"cos(-x) = 1/2
and cos(-x) = cos(x)
therefore,
cos(x) = 1/2"

slate saddle
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oh

limber quartz
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wouldn't be that

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because when k=0, x = pi/2, which isn't going to work

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use what I just wrote

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(typed)

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the last line

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to draw a right triangle with angle x

slate saddle
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doing it now

limber quartz
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or, you know

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you don't need to draw the picture if you have the unit circle memorized

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what angle theta (in our case, x) has a cosine value of 1/2?

slate saddle
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pi/3

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and

limber quartz
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cosine

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not sine

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pi/3 --> 60 degrees

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oh!

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you are right actually

slate saddle
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so pi/3 and 5pi/3

limber quartz
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yes.

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in the interval [0,2pi)

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very good job

slate saddle
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thank you

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would

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or rather

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is x already in the interval [0,2pi) ?

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Or would I need to solve that as well?

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wait a min

limber quartz
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it is in the interval.

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you made it so

slate saddle
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aha

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I see

limber quartz
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you found the most accute angle that had that trig ratio

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or as we would say, the reference angle/triangle

slate saddle
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So there are only two solutions within the interval [0,2pi)?

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As those are the only ones we found.

limber quartz
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I wish I could draw you the picture

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you would draw the reference triangle in Q1

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and that angle in Q1 would have its terminal side as the hypotenuse

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I hope that is obvious

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but then, since cosine is also positive in Q2 (positive is desired because since we had cos(x) = +1/2)

slate saddle
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ah

limber quartz
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we lay the same reference triangle in Q2 on the -x axis

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and the new angle terminates at that hypotenuse

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but notice that would just be 180 degrees - x

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or pi - x

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that is,

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pi/3, pi - pi/3 = 2pi/3

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oh

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you got the other angle wrong

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I think

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oh cheese and crackers

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I got it wrong

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I'm thinking of sine again!

slate saddle
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lol

limber quartz
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don't orient the second triangle along the -x axis

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flip it OVER the +x axis

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then the angle you desire is 2pi - pi/3

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6pi/3 - pi/3 = 5pi/3

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so you are correct

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yes

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sorry

slate saddle
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Ah

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So

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The 1st triangle is in Q1 and I would flip it into Q2?

limber quartz
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nah

slate saddle
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Q2 being the +x axis?

limber quartz
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flip it into Q4

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because cosine has positve values in Q1 and Q4

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and that is because the horizontal components of the right triangles are positive in Q1 and Q4

slate saddle
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yep

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WHAT AM I THINKING Q2

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I meant Q4

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yes

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got it

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Thank you very much for your assitance and help!

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This is a very through conversation.

limber quartz
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hang on

slate saddle
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oh dear there's more?

limber quartz
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Just a picture

slate saddle
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ah

limber quartz
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in the unit circle

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the hypotenues is always "1 unit"

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any triangle can have all its sides scaled up or down by the same amount to make this so.

slate saddle
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got it

limber quartz
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it's only the x and y-components of the triangle that have a sign (positive or negative). The hypotenuse is always positive.

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so the rules for flipping the reference triangles can be visualized picorally by just noticing where the sine and cosine values are the same result for congruent triangles in different quadrants

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the signs of the sides are different in different quadrants, obviously

limber quartz
slate saddle
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hmmm

limber quartz
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I added the quadrant labels and the signs of cos, sin, tan (in that order) for each quadrant

slate saddle
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Thank you very much!

limber quartz
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no problem

slate saddle
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I wish you a great rest of your night or day (wherever you are in the world).

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Cheers!

#

.close

obtuse pebbleBOT
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timid silo
#

anyone?

obtuse pebbleBOT
timid silo
#

Calculate the length of segment AD, if |OA| = 50 cm, |OB| = 28 cm, and [OCI] = 7 cm.

#

into english ^^

alpine bison
#

just use the intercept theorem

obtuse pebbleBOT
#

@timid silo Has your question been resolved?

robust raven
#

to jest dosyć łatwe, pomyśl o tw. Talesa 🙂

#

najpierw obliczasz OD, a potem sumujesz

timid silo
#

A to

alpine bison
timid silo
#

“• W troikacie ABC o bokach drugosci: | AB| = 18 cm, | BC| = 24 cm, | AC| = 20 cm poprowadzono dwusieczna kata ABC, która preciela bok AC w punkcie D. Oblicz dugosci odcinkow AD i CD.”

#

Strasznie nie kumam o co tu chodzi

robust raven
#

oznacz sobie szukane boki jako x i y

obtuse pebbleBOT
#
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robust raven
#

.reopen

#

wteyd masz x + y = 20

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następnie:

robust raven
obtuse pebbleBOT
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timid silo
#

??

obtuse pebbleBOT
timid silo
#

Tak

robust raven
#

jestem , potem piszesz x / y = x / (20-x) = 18/24 = 3/4

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obliczasz 4x = 3(20-x) <=> 7x = 60 itd

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x = 60/7 , y = 80/7,

obtuse pebbleBOT
#

@timid silo Has your question been resolved?

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ocean arrow
#

hi

obtuse pebbleBOT
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crude coral
obtuse pebbleBOT
crude coral
#

why is my answer in negatives? it should be 10/9

#

the question is $$\int^1_0 \frac{3x}{\sqrt{4-3x}}$$

warm shaleBOT
#

JustToPro