#help-36

1 messages · Page 205 of 1

fallow compass
#

What do you mean by that?

silver cargo
#

exactly what I said: explicitly show the values of s and q too

#

from what I can see, unless I missed some blindingly obvious gotcha, you only found p and r = -1

fallow compass
#

Since q=-s
I don't think I need to find s and q

#

Oh wait

desert mantle
#

p+q+r+s=(p+q)+(r+s)=r+p

fallow compass
#

It should be -p = r+s

desert mantle
#

so finding s and q is not necessary

fallow compass
#

Wrong vietta formula

#

Dammit

desert mantle
#

oh true

silver cargo
#

ah, fair enough then

desert mantle
#

unlucky

fallow compass
#

Luckily the answer is still -2

silver cargo
#

I'm just making sure you avoid any instance of people asking "but what about the other two variables?"

#

but if you have a solid enough reasoning around that, that's completely fair, and my bad for not realizing it

fallow compass
#

So the answer is correct yes?

#

Then, thank you

#

.solved

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#
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drowsy viper
#

i need help

final saddleBOT
drowsy viper
#

1: random variable x has a probability density function of 3/8 (x^3-5x^2+6x) 0≤x≤2
A: sketch graph for this function
B: show that f(x) is a valid probability density function
C: to 3 significant figures find
i: its mean
ii: its mode
iii: P((1/2) ≤x≤ (3/2))

idk how to do part C at all, this was the question i was given

barren hound
#

mean should be $\int x f(x)\dd{x}$ right?

soft zealotBOT
#

〈 kitten | teacup 〉

drowsy viper
#

i did i and iii, could i use the quadratic formula for ii? doing it rn seems like the right thing to do idk tho, dont have my textbook to check lol and ive gotta do this by tmrw

barren hound
#

yeah you just want to maximise f(x)

#

quadratic seems fine

drowsy viper
#

okay cool, thanks, you can like end the thingy now idk how to do it

barren hound
#

.close

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dawn plover
final saddleBOT
dawn plover
#

Sorry its not in english ill try to translate best i can

#

So basically a, b, c are any numbers that fit a + b + c = 1

#

and we have to prove the equation is correct

tiny kraken
#

any numbers? or like for positive numbers

dawn plover
#

sorry

#

positive number

tiny kraken
#

These problem seems like typical AM-GM type for positive number so I just want to check

dawn plover
#

yeah positive

tiny kraken
#

But you can predict when the equality hold and try to AM-GM

bronze grove
dawn plover
#

well i assume you cant

#

since they are different letters

bronze grove
#

then u will get value of a that proves the equation a + b + c = 1 is correct

bronze grove
#

that a = b = c

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and we will just a >= 1/3 after solving the equation

dawn plover
#

how do you know it so fast

bronze grove
#

?

dawn plover
#

wait so what do i do

#

a = b = c = 1/3?

bronze grove
#

put b = a

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and c = a

bronze grove
dawn plover
#

so b = a and c = a

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got it

bronze grove
#

yes

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but im not 100% if u can do it that way but worth a try ig

dawn plover
#

ok lemme try

tiny kraken
#

dafuq that is not how you're supposed to do a inequality problemmmmmm

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but ehhh

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just try it ig

dawn plover
#

ive never done smth like this

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ok

#

i have 3/(3a^2 + 2a) => 1/3a^2

bronze grove
dawn plover
#

i am

#

ok i got it

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a => 0 and a => 1/3

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ok well answer can be 1/3

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but i strongly belive its incorrect for some reason

bronze grove
#

yea me too

#

i think we need to put c = 1-a-b

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but the calculation is too long

dawn plover
#

well

#

if you want i got another problem

#

we can ditch this one

bronze grove
#

suree

dawn plover
#

this is last one i have unsolved

#

we have to find all combinations of a b c m n

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and they are natural numbers (1, 2, 3, 4...)

bronze grove
dawn plover
#

what is substitute

bronze grove
#

putting value of something in another equation

#

so u get the value of b from first equy

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and put it in second equi

dawn plover
#

im not sure if i can do that

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probably can

#

ok but how does that help me

deft terrace
#

so first square the expression a+b+c = 1

#

and find the value of the sum of their squares

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then put them greater or = 0

proper dagger
deft terrace
#

then get an inequality using a.m h.m inequvalence

dawn plover
#

im on different question tho

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since i couldnt get the first one with help

dawn plover
#

so do we do 1st question or 2nd

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i need to solve at least one today

deft terrace
#

i did 1st

dawn plover
#

ok

deft terrace
#

lemme do the second

dawn plover
#

ok

dawn plover
deft terrace
dawn plover
#

dont give me solutions right away tho

deft terrace
dawn plover
deft terrace
dawn plover
#

noooo

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i want to work on the 2nd one ill try 1st one later on

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with the square you said

bronze grove
#

didn't think of that

deft terrace
dawn plover
#

sum of squares would just be the same tho

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= 1

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im getting off tracked

dawn plover
#

ok so what i think here

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try getting value of b from first one

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and from second one make c^m x c^n so we can also put in value of c^m from first one?

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not sure how that helps in the long run

bronze grove
#

so when we get the second equation

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what are u getting?

dawn plover
#

what

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what is your question

bronze grove
#

when u put b in second equation

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whaf are u getting

dawn plover
#

you get rid of b's

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yeah you do

deft terrace
#

find the value of a and b

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in the terms of c

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then focus on what you get for a

dawn plover
#

why value of a

deft terrace
#

it will be of the form c^m(5-c^n)/24

dawn plover
#

isnt b just enough

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i have no clue how you got this

deft terrace
dawn plover
#

im not looking for solution

#

just help

deft terrace
#

i am

deft terrace
#

step 1 : find a

dawn plover
#

from first one?

deft terrace
#

step 2 : make sure that a is positive and natural

#

done

dawn plover
#

ok well that didnt help 🙁

deft terrace
deft terrace
dawn plover
#

b = c^m - 5a

deft terrace
#

then

#

you use this value in the second equation

dawn plover
#

yeah

deft terrace
#

what do you get then

dawn plover
#

shit

#

well i got 5c^m - 24a = c^m x c^n

deft terrace
#

now find the value of a and tell me

dawn plover
#

i have no clue how to do that but wait

#

well i know if we can still use unknowns

deft terrace
#

take the terms with a on one side

#

and the terms with c on the other

dawn plover
#

yea

deft terrace
dawn plover
#

i get

#

a = (c^n x c^m - 5c^m) / -24

#

eh

deft terrace
#

good

#

now in the bracket above multiply the -1 from the denominator

#

what do you get

dawn plover
#

what

#

my english is limited in terms of math

deft terrace
dawn plover
#

we can just put it in front

#

you still get negative either way

deft terrace
#

now take that - inside the brackets

dawn plover
#

why

#

dont i get -c's then?

deft terrace
deft terrace
dawn plover
#

ok 🙁

bronze grove
dawn plover
#

a = 5c^m-(c^n x c^m)/24

bronze grove
#

lets start by putting a = 1

deft terrace
deft terrace
#

no

deft terrace
dawn plover
#

for some reason i assume it has to be interval as answer

deft terrace
#

take it common

dawn plover
#

its - and multiplication

#

i cant just do minus first

#

substraction

deft terrace
dawn plover
#

what

deft terrace
#

do you know 2X3+2X5 = 2X8

dawn plover
#

???

#

its not

deft terrace
dawn plover
#

2x^3 times 2x^5 would be 2x^8

bronze grove
deft terrace
#

it will make it c^m(5-c^n)/24

dawn plover
#

i still dont get what u want me to do

deft terrace
#

now we know a is +ve

deft terrace
bronze grove
deft terrace
dawn plover
#

wait i think i understood

deft terrace
#

c^m is always +ve

#

so is 24

#

so 5-c^n >0

#

now we find all possible value

#

s

dawn plover
deft terrace
dawn plover
#

um yea

#

but still 2 unknowns

deft terrace
dawn plover
#

waiiiiiittt

deft terrace
#

i am a pretty patient person

dawn plover
#

c^n < 5

deft terrace
#

now tell me all possible combinations of (c,n)

dawn plover
#

5^1

deft terrace
#

nope

dawn plover
#

oops

#

mb

#

MY BAD

#

wait does it have to be interval?

deft terrace
deft terrace
dawn plover
#

ok so

#

c = 2 n = 2

#

c = 1 n = any positive natural

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well

deft terrace
dawn plover
#

another issue

#

nvm no issue

dawn plover
#

so c can be 1 and 2

#

n can be 1, 2 and any number

#

depends on scenario

deft terrace
#

well we get
c=1 n= any natural no.
c=2 n=1,2
c=3 n=1
c=4 n=1
right

dawn plover
#

why first one no

#

1^1808080 is still one

deft terrace
dawn plover
#

you said no

deft terrace
dawn plover
deft terrace
dawn plover
#

oh ok

deft terrace
#

no.=number

dawn plover
#

ok

deft terrace
#

now second task is to make a and integer

dawn plover
#

ohhh

deft terrace
#

so no fractions

dawn plover
#

its starting to make sense slowly

#

c = 1, 2, 3, 4

dawn plover
deft terrace
dawn plover
#

input any?

deft terrace
#

by taking the case of c and n found above and using diffrent value's of m

dawn plover
#

so do i put a as 1

deft terrace
deft terrace
dawn plover
#

wha

#

what do you want me to do then

deft terrace
#

never mind

#

continue

dawn plover
#

a = 1?

deft terrace
#

c=1 now what

#

nope

#

a was a = c^m(5-c^n)/24

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right

dawn plover
#

yeah

#

i got it already now

deft terrace
#

so now if we use c as 1 we get a= 1/24

#

is that a natural number

dawn plover
#

not really no

deft terrace
#

so c cant be 1

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now try c as 2

dawn plover
#

depends on n

deft terrace
#

use n=1 first

dawn plover
#

i am

#

i just said it does

deft terrace
#

check if a value of m is possible

dawn plover
#

wait this is gonna sound stupid

#

can i multiply 2^m x 3

deft terrace
#

yes

#

but it will change nothing

dawn plover
#

to get 6^m?

#

is that a thing?

deft terrace
#

uhh no

dawn plover
#

yeah ok so i think well

deft terrace
#

both should have the same power

dawn plover
#

2^m x 3 / 24

#

we dont got a

#

oh nvm i got it

#

mb

#

m can be 3

#

m can be 4

deft terrace
dawn plover
#

yea

deft terrace
#

so we got m>3 for n=1 and c=2

#

now check for the other values

dawn plover
#

c = 3?

deft terrace
#

ye

dawn plover
#

not 3

#

unless its smth above 5

deft terrace
dawn plover
#

no 4

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a = 6.75 then

#

oh wait i can finish this on my own now

#

well i think with enough time i could have finished this one ty for help

deft terrace
dawn plover
#

4 is possible too btw

dawn plover
#

should we try 1st problem real quick?

deft terrace
#

c cant be 4

dawn plover
#

nvm

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my bad

#

its 4 x 1

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i forgot

deft terrace
dawn plover
#

i do

#

i mean i have 3 more problems but idk if i can do them all today

#

ok so

#

a + b + c = 1 would just be all squares = 1

deft terrace
dawn plover
#

1^2 = 1

deft terrace
#

and take 2ab + 2bc + 2ca the other side

deft terrace
#

expand this

dawn plover
#

can you even

#

you cant

deft terrace
deft terrace
#

now solve from here

dawn plover
#

ok this is pointless for me then

#

ive never seen this expantion thing

#

ok anyways i dont think i could do this one so im done for today, its getting late so thanks for your time

#

.close

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#
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radiant carbon
final saddleBOT
radiant carbon
#

how do i do this

abstract bramble
zinc geyser
#

what do each of those circle represent?

radiant carbon
#

small circle is german

stiff bear
#

so not german so c or d

#

c?

radiant carbon
#

nah

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its A

#

lol

#

idk why

stiff bear
#

but german isn’t shaded in

#

wait what

#

why would german be the small circle

#

spanish would be small circle

radiant carbon
#

.close

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pastel karma
#

How should i start this, i dont have much understanding

radiant cedar
pastel karma
#

I just forgot how to implement it

#

Im reviewing for a test

radiant cedar
#

$f(x) = a(x - h)^2 + k$

soft zealotBOT
#

kizzyyy

pastel karma
#

Damn

radiant cedar
#

does this ring a bell?

pastel karma
#

Im washed

pastel karma
#

Looks familiar

radiant cedar
#

what does (h,k) represent?

pastel karma
#

I dont know

#

Something to do with a parabola

radiant cedar
pastel karma
radiant cedar
#

anyway, you're given the vertex

#

so likke can you fill in

radiant cedar
#

what would the new f(x) look like?

pastel karma
#

F(x) = a (x-5)^2+9?

radiant cedar
#

close

#

what is h?

pastel karma
#

X coordinate of vertex

radiant cedar
#

i mean what is it

#

in your case

pastel karma
#

-5

radiant cedar
#

,, f(x) = a(x - (-5))^2 + 9

soft zealotBOT
#

kizzyyy

radiant cedar
#

is it

pastel karma
#

Oh

#

No it isnt

radiant cedar
#

anyway so

#

since -(-5) = 5

#

it's f(x) = a(x + 5)^2 + 9

#

all u have to do now is find the value of a

#

you'd use this particular fact to aid you with that:

"quadratic function.... passes through the point (-7,-15)"

spiral skiff
#

Hi, I would like to know if question b requires a module, if so, how does this affect the result?

radiant cedar
#

someone's already using this channel

pastel karma
spiral skiff
#

Okk sorry

pastel karma
#

How would i find this again

pastel karma
radiant cedar
#

this passes through (-7,-15)

#

which means u can plug it into your equation and solve for a

#

y = -15 and x = -7

pastel karma
#

OH

#

-15=a (-7+5)^2 +9

#

?

radiant cedar
#

-15 but yes

pastel karma
#

Ok i got it right

#

Thanks

#

.close

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rugged merlin
#

Why does this hold [
\ceil{\4ab} = \floor{\4{a+b-1}b}
]

soft zealotBOT
bronze grove
rugged merlin
#

I do, yes

bronze grove
#

by dividing it in three parts

rugged merlin
#

yes, [
\floor{\4ab+1-\41b}
]

soft zealotBOT
bronze grove
#

do u know pascal's identity?

rugged merlin
#

i do not

bronze grove
rugged merlin
#

Looking it up, it seems to be a combinatoric identity; how does it apply in our case?

bronze grove
#

lemme think of easier way wait

#

it always holds for a = b

rugged merlin
#

I mean I managed to prove the case where a is a multiple of b

robust mulch
#

lex lisayay

bronze grove
drowsy epoch
rugged merlin
#

oh hi garlic! nice new pfp :D

#

ok yeah i think i figured it out

#

using the division algorithm, if $r = 0$, we have $a =qb$: [
\floor{\4{a+b-1}b} = \floor{q + \4{b-1}b}
]
and since $b\in\Z_{\ge1}$ that term will always be between 0 and 1, hence [
\floor{\4{a+b-1}b} = q
]

soft zealotBOT
rugged merlin
#

if $r\ne 0$ then [
\floor{\4{a+b-1}b} = \floor{q+1+\4{r-1}b}
]
Since $1\le r< b$ the term $r-1$ satisifes $0\le r-1< b-1$. Hence [
\floor{\4{a+b-1}b} = q + 1
]

#

well at least i think thats right?

bronze grove
#

yess

soft zealotBOT
tiny kraken
#

emojigg_Huh one question, how can you make sure r-1>0 are a and b integers or smth

final saddleBOT
#

@rugged merlin Has your question been resolved?

rugged merlin
#

it says [
a= bq + r, \q 0 \le r < b
]

soft zealotBOT
rugged merlin
#

r being bounded that because, conceptually, division is just partitioning a number a into b cells, and r is the leftover component. If it was greater than b then it would mean the divisor could have fit in at least one more time.

tiny kraken
#

But we're proving for real number no?

rugged merlin
#

for my case i needed this for integers a and b, so not in my case no

#

also i mean i think you can come up with a counterexample that disproves it for the real case

#

,w ceiling(9.5/3) = floor((9.5+3-1)/3)

tiny kraken
#

Ehhh I thought we're proving for real numbers

rugged merlin
tiny kraken
#

Then I think your proof looks good. Or I'm not able to spot any flaw

rugged merlin
#

yeppie

#

have a good one

#

.close

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harsh stratus
#

hello requesting help with this problem

final saddleBOT
harsh stratus
#

the area of a sphere increases with 28 cm^2 per minute

#

with which speed does the volume of the cube increase when the radius is 6.5 cm

#

let me show my work

tired walrus
#

hol up

drowsy epoch
tired walrus
#

the area of a sphere increases with 28 cm^2 per minute
with which speed does the volume of the cube increase when the radius is 6.5 cm

#

volume of the cube?

harsh stratus
#

oh my apologies

#

the volume of the sphere

#

r(t)

drowsy epoch
terse crypt
#

😔

harsh stratus
#

hmm

#

but I don't have a time

#

why the derivative with respect to time

drowsy epoch
#

it says the area of a sphere increases per minute

harsh stratus
#

oh okey fine

drowsy epoch
#

so you wanna know the rate of change in terms of time, not distance

harsh stratus
#

we're searching for

#

dv/dr

#

I assume

drowsy epoch
#

speed would more like be change in time

harsh stratus
#

one second

#

heading to the grocery market now

final saddleBOT
#

@harsh stratus Has your question been resolved?

signal cipher
#

you are an it

#

a fucking it

severe canyon
#

And so?

#

It's not relevant

severe canyon
final saddleBOT
signal cipher
# severe canyon !redir

oh my bad dude i thought this was discussion channel because it was open by default. Sorry

severe canyon
#

Don't worry

final saddleBOT
#
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pearl solar
final saddleBOT
pearl solar
#

Hi im very confused on this proof

#

like

#

I understand that the cartesian product of A and B, order matters

#

I understand the idea of an ordered pair

#

but

#

Im unsure how the double power set comes in here

worldly mesa
#

x in P(P(A)) means that x is a set of sets which are all subsets of A

pearl solar
#

yh this is mind boggling

worldly mesa
#

so {{1}, {2}} is a set of sets each being a subset of {1,2}

pearl solar
worldly mesa
#

this is a formal definitions

pearl solar
#

OHHHH

#

I SEE

worldly mesa
#

how you want to remember AxB is as the set of ordered pairs of elements (a,b) where a in A and b in B

#

and all this proof really does is demonstrate how from the axioms we can build this set

pearl solar
#

so what the first power set does is, say for (1,2) u get {1}, {1,2} sets, the seocnd power set {{1}, {1,2}} ?

worldly mesa
#

yeah

#

saying x in P(A) mean x is a subset of A

#

so x in P(P(A)) means x is a subset of P(A)

pearl solar
#

ahh i understand it now catthumbsup

#

tysm

pearl solar
#

alr ty

#

.close

final saddleBOT
#
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rugged merlin
#

\def\blah{\xrightleftharpoons{\hspace{0.667cm}}} [ \int_{-\infty}^\infty g_1(t)g_2^(t-\tau)\dd t \overset{\mathcal F}\blah G_1(f)G_2^(f) ]

latent dragon
#

what does the left right harpoons mean here?

rugged merlin
latent dragon
#

i’ve never seen this notation before. hmmCat

#

*in math atleast

proper raptor
soft zealotBOT
rugged merlin
#

Anyways, I am trying to make sense of what this is meant to be telling me

sick roost
sick roost
#

G is the fourier transform of g here

proper raptor
#

ahh ok and * is the complex conjugate ?

rugged merlin
#

indeed

sick roost
rugged merlin
sick roost
#

In signal processing, cross-correlation is a measure of similarity of two series as a function of the displacement of one relative to the other. This is also known as a sliding dot product or sliding inner-product. It is commonly used for searching a long signal for a shorter, known feature. It has applications in pattern recognition, single par...

#

Its very similar to a convolution in function

rugged merlin
#

It seems highly reminisicent of what convolution is to be honest

#

yeah

#

exactly

#

so there are three things: convolution, cross correlation and autocorrelation

sick roost
#

Autocorrelation is like a convolution with itself

#

I mean the name literally means ‘correlation with itself’

#

Actually, the cross-correlation is precisely a convolution but it takes one functions conjugate

rugged merlin
#

so cross correlation and convolution resolve to the same thing with real valued functions?

sick roost
#

Well, the time domain is slightly different

rugged merlin
#

this is highly confusing

#

i cant conceptualise the difference between the two

sick roost
#

I believe the time domain is negative over real functions

#

Thats how theyre related

#

I might be insane rn, lemme check something rq

#

Right, because the convolution is a function of t, but the cross correlation is of tau

#

So the time domain is negative for reals

#

They are otherwise the same here

#

We know this directly via a simple u sub

#

u=t-tau

soft zealotBOT
sick roost
#

We should take the convolution d\tau

#

Not dt

#

Its just symbol changing here

rugged merlin
#

well yes

#

then it should be dt

#

as i wrote

sick roost
#

But its important we keep tau and t consistent across the domains

#

This is probably where the confusion is

#

One sec

rugged merlin
#

the thing on the right will have $\exp(j2\pi f\tau)$ as its kernel

soft zealotBOT
rugged merlin
#

uhh

sick roost
#

the convolution should be put
$$(f*g)(t)=\int_\mathbb{R}f(\tau)g(t-\tau)d\tau$$

soft zealotBOT
#

Cycadellic

sick roost
#

is what i meant

#

compare with the cross-correlation
$$(f\star g)(\tau)=\int_\mathbb{R}f^*(t-\tau)g(t)dt$$

soft zealotBOT
#

Cycadellic

rugged merlin
# soft zealot

yeah im trying to derive this formulation starting from convolution

sick roost
#

so, take f's conjugate, convert to negative time domain, then they are related

sick roost
rugged merlin
#

aight

sick roost
#

choose f^*, g for convolution

#

as opposed to f, g

#

apply convolution theorem, the result should follow immediately

rugged merlin
#

,, \int_\R f(\tau)g(t-\tau)\dd t \overset{u= t-\tau}= -\int_\R f(t-u)g(u)\dd u

#

i think i did that correctly

sick roost
#

now we want f^* instead of f

soft zealotBOT
rugged merlin
sick roost
#

no, just take a new f=h^* or something like that

#

or just plug in f^* directly

rugged merlin
#

right

#

you end up with [
-\int_\R f^*(t-u)g(u)\dd u
]

soft zealotBOT
rugged merlin
#

what this is like

sick roost
#

wait

#

lose the -

#

u = t-\tau \implies du=dt

rugged merlin
#

not t

#

i typo'd

sick roost
#

we want u here

rugged merlin
#

yeah im saying u = t- \tau implies du = -dtau

sick roost
#

oh i see

#

right

sick roost
#

should be taken d\tau

sick roost
rugged merlin
#

well i mean it seems like [
(f\star g)(t) = f^*(-t)*g(t)
]

#

urgh

soft zealotBOT
sick roost
#

yeah

#

and thus $$=(f^**g)(-\tau)$$

soft zealotBOT
#

Cycadellic

sick roost
#

and so it is a modified convolution

rugged merlin
#

oh i see

#

so applying the convolution theorem

#

wait how would that -tau affect our result from the convolution theorem

#

or would it

sick roost
#

well, continue writing it out

#

what does the convolution theorem say about $(f^**g)(-\tau)$, now?

soft zealotBOT
#

Cycadellic

rugged merlin
#

wait but before that

#

im a bit unconvinced still with that formulation: [
(f^**g)(-\tau) = \int_\R f^(t)g(-\tau - t)\dd t \0{red}\ne \int_\R f^(t)g(t-\tau)\dd t
]

soft zealotBOT
rugged merlin
#

the latter being the actual correlation

sick roost
#

right, i might have said something stupid

#

ill be more precise

final saddleBOT
#

@rugged merlin Has your question been resolved?

sick roost
#

oh wait im literally going insane

#

LOL

#

theyre the same theorem here

#

but with G_2^* instead of G_2

#

no u-sub necessary

#

just swap t and tau, and take G_2^* instead of G_2

rugged merlin
#

Oh I see

#

Right ok

#

Yeah well that was easier tahn we made it out to be

sick roost
#

the u-sub with the negative sign made me lose my mind lol

rugged merlin
#

😅

#

Still a bit confused by what convolution and correlation are really saying

#

I mean i get convolution but not this

sick roost
#

this is very similar to the statistical cross-correlation with some extra assumptions

#

stats takes the expectation of it, is the real difference

#

i think this image on the wiki does a good job explaining exactly whats going on

rugged merlin
#

But didnt you see autocorrelation is the same signal

#

So why are f and g diggerent

sick roost
#

well, hang on, the autocorrelation is on f\star f, and then g\star g

rugged merlin
#

I see

#

Well visually

#

It seems like correlation is convolution but without the flipping that occurs bcuz of the g_2(t-tau)

sick roost
#

specifically the correlation is a convolution, but with a sign flip and conjugate on the first function

rugged merlin
#

Seems like ghe conjugation is irrelevant here

#

Considering these seem to be real

sick roost
#

over R, they really come out very similarly, except the convolution is commutative, which the cross correlation flips the domain on commutation

final saddleBOT
#
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rugged merlin
#

.reopen

sick roost
#

.reopen

final saddleBOT
sick roost
#

you forgot to react to the bot

rugged merlin
#

Yeah soz

rugged merlin
#

Ok wait

sick roost
#

in the time domain

rugged merlin
#

I think we need to establish something

sick roost
#

we showed that $$(f\star g)=(f^*(-t)*g)$$

soft zealotBOT
#

Cycadellic

rugged merlin
sick roost
#

well, hold on

#

convolution is in t-domain, correlation is in tau-domain

heavy mural
#

Hi

#

Is there someone here to help

rugged merlin
#

It just doesn't check out with the convolution theorem

#

But im eyeballing it rn

sick roost
heavy mural
#

I got the answer D for this is that right

sick roost
heavy mural
#

Oh ok

rugged merlin
sick roost
#

they are both time domains, we should probably think of the axes as the same there

rugged merlin
#

Correlation seems to be just 1. Flip t and tau 2. Chug a * from the conv formula

rugged merlin
sick roost
#

$$(fg)(t)=\int_\mathbb{R}f(\tau)g(t-\tau)d\tau$$
let $u=t-\tau\implies du=-d\tau$
$$(f^
(-t)g)=-\int_\infty^{-\infty}f^(u-t)g(u)du$$

#

and so, $$f^*(-t)*g=f\star g$$

soft zealotBOT
#

Cycadellic

#

Cycadellic

sick roost
#

because u = infinity implies tau = -infinity
so thats how we deal with the minus sign here

final saddleBOT
#

@rugged merlin Has your question been resolved?

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solar crest
#

hi im a bit confused, if i have a matrix m x n why is it that:
if m > n, there is no way that it's surjective?

solar crest
#

and why is it that if m < n, there;s no way it's injective?

#

i wanna understand the logic behind i t

static plinth
#

i would say it’s easier to understand that logic if you know that any matrix represents a linear transformation

#

are you familiar with that idea?

ivory sandal
static plinth
#

matrices/linear maps cannot grow dimensions

#

it can only keep its current dimension or reduce it

solar crest
solar crest
solar crest
#

if i have r3 --> r2

#

then it's surjective

#

but if it have r3 --> r3 it cannot be?

static plinth
#

well it can still be surjective

#

because we have an input space of dimension 3 and an output space of dimension 3

#

there would exist some linear map that would retain its "size"

ivory sandal
static plinth
#

but take T: R^3 -> R^4

#

theres just no possible linear map you can make such that the codomain is all of R^4

#

thats the idea of how linear maps cannot grow dimensions

final saddleBOT
#

@solar crest Has your question been resolved?

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main mirage
#

Hi. I'm not sure how to use the hint in this question, can anyone help?

The function $\delta(x)$ refers to the dirac delta function. That is:
[
\int_a^b \delta(x)dx = \begin{cases} 1 \quad \text{if: } 0 \in (a, b) \
0 \quad \text{otherwise.}
\end{cases}
]

soft zealotBOT
#

Good Luck HSC Students!

rocky tusk
#

did you look at integrating delta(ax)?

main mirage
#

Or did you literally mean integrating delta(ax) straight from the start?

rocky tusk
main mirage
#

I'm not sure how you would do that sorry.

rocky tusk
soft zealotBOT
rocky tusk
#

u = ax

final saddleBOT
#

@main mirage Has your question been resolved?

main mirage
# rocky tusk $\int_a^b \delta(ax) \phi(x) \dd{x}$

Hmm. Still a little unsure, but this is what I am thinking of:
[ \int_{-\infty}^{\infty}\phi\left(x\right)\delta(x)dx=\phi\left(0\right) ] [ \int_{-\infty}^{\infty}\phi\left(au\right)\delta(au)adu=\phi\left(0\right) ] [ \int_{-\infty}^{\infty}\phi\left(x\right)\delta(x)dx=a\int_{-\infty}^{\infty}\phi\left(au\right)\delta(au)du ] If we let $\phi(x) = 1$, then: [ \int_{-\infty}^{\infty}\delta(x)dx=a\int_{-\infty}^{\infty}\delta(au)du ] Is this on the right track?

soft zealotBOT
#

Good Luck HSC Students!

main mirage
#

I'm guessing there should really be |a| at the front of the integral, because when we make the substitution x = au, we don't want the bounds of the integral to flip?

#

However, does this mean the integrals cancel out? What is the justification for that?

final saddleBOT
#

@main mirage Has your question been resolved?

final saddleBOT
#

@main mirage Has your question been resolved?

pliant shore
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main mirage
final saddleBOT
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jagged flare
final saddleBOT
jagged flare
#

lemme send the hints

fiery bluff
jagged flare
#

599=||This is just a statement about distances to line OH; ignore the areas.||
152=|| You can replace line OH with any line through the centroid G.||
598=|| Without loss of generality, B, C lie on the same side of the line. Let D be the midpoint of BC.||
545=||Use the fact that AG = 2GD.||

#

yeah im stuck on how i would prove AX=BY+CZ

#

i havent used the last hint yet so its prolly related to it

fiery bluff
#

pigeonhole principle - two sides of the line, 3 points so two points are on the same side

jagged flare
#

yeah i just realized that

fiery bluff
#

You have to use point d somewhat

#

Consider ||distance from d to the line||

jagged flare
#

huh wait ok hold on this is interesting

#

DK is half of AX

#

since BY//DK//CZ, and D is the midpoint of BC, by similarity DK=(BY+CZ)/2 <=> AX=BY+CZ, QED?

#

ok yeah i think this is it

#

thank you so much!

#

.solved

final saddleBOT
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dusky isle
final saddleBOT
dusky isle
#

Over here, is the purpose of finding a general equation?

#

I dont understand what we are exactly doing here, the idea is to use differential equations:

#

Sorry if the handwriting is bad

final saddleBOT
#

@dusky isle Has your question been resolved?

final saddleBOT
#

@dusky isle Has your question been resolved?

final saddleBOT
#

@dusky isle Has your question been resolved?

final saddleBOT
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timber moth
#

Hey, extremly rough draft as i just learned little o notation 5m ago but is any of this correct

timber moth
#

Im mostly asking for the potentially botched use of some sort of triangle inequality with h1, and h2

#

forgot to say by using squeeze theorem we can bind g(h) between 0 and 3h at the end there **

#

.close

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ivory lynx
#

hi guys, dumb question

final saddleBOT
ivory lynx
#

its mathematical induction

#

how did we get (k +1)(k + 2) in the thrid step?

opal plinth
#

This, but with n = k+1

ivory lynx
#

OH

#

i need

#

i see**

#

k + 1 = k+ 1(k + 1 + 1)

#

right right

#

thank you

#

🙏

#

.close

final saddleBOT
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final saddleBOT
onyx peak
#

<@&268886789983436800>

final saddleBOT
#
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latent dragon
#

spam?

onyx peak
latent dragon
#

lotsa those these days. Sigh

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warm python
final saddleBOT
warm python
#

$\int_{0}^{1} \int_{0}^{1} 2x^2y+Cy^5=1$
\
Solving this will give me the value of C

tiny gorge
#

no

#

why should that integrate to 1?

warm python
#

oops

tiny gorge
#

the joint PDF doesn't involve integrals..

warm python
#

over (0,1)*(0,1)

tiny gorge
#

yes

#

integrate over the full domain to get 1

soft zealotBOT
tiny gorge
#

yep

warm python
#

lemme solve it rq

#

C=2

tiny gorge
#

i get a different answer

#

,w integrate (2x^2 y + 2y^5) for x = 0 to 1 and y = 0 to 1

soft zealotBOT
warm python
#

Okay, lemme send my steps

#

you've got to be kidding me

warm python
tiny gorge
#

haha numbers are hard

warm python
#

😭

warm python
#

$\int x \cdot 2x^2 dx = x^2$

soft zealotBOT
tiny gorge
#

😭

warm python
#

This messed up eveyrthing

#

and cost me 5 marks I think

tiny gorge
#

yea stupid calculus mistakes are the source of many problems in higher level courses, alas

warm python
#

Sorry, more than help, I need to rant

tiny gorge
#

nw, it happens to all of us

warm python
#

and while calculating something I got 30 instead of 35 ( cost me 5 marks)

tiny gorge
#

harsh grading!

warm python
#

anyways, just to be sure

tiny gorge
#

this kind of crap is why i always stayed until the end of every exam even if i finished early
double and triple check everything, i nearly always found some stupid arithmetic mistake

warm python
# warm python

here once I get C, I integrate wrt y from 0 to 1 fo get F_X(x) and similarly for F_Y(y)

tiny gorge
#

well not from 0 to 1

#

you want to find $F_{X,Y}(0.8, 0.6)$

soft zealotBOT
tiny gorge
#

so you would integrate from x = 0 to 0.8, and y = 0 to 0.6

warm python
#

I also need F_X(x) and F_Y(y)

tiny gorge
#

oh ok i overlooked that, lemme go reread

tiny gorge
#

(this gives you the marginal pdf's, not the marginal cdf's)

warm python
#

t

#

tq

tiny gorge
#

yw

warm python
#

.close

final saddleBOT
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stone cradle
#

its on german so dont wonder. My question to this is what does it means when s = -1.5? like a = -1.5c, but what does that means ?

terse crypt
#

${\vec{a} = -1.5\vec{c}}$

soft zealotBOT
terse crypt
#

it means that vector a is in the opposite direction of vector c

#

and its magnitude is 1.5 times that of c

stone cradle
#

oh, so if c is 10m long a is 15m long ?

#

wait

#

no

terse crypt
#

and is pointing in opposite direction

stone cradle
#

and what would it be if its 1.5c

#

same direction?

terse crypt
stone cradle
#

oh got it thanks

#

.close

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stone cradle
#

used an translator and idk if its the right topic

lines in space,
the vector parametric equation

anyways, my question is how do i use that with an line in an 3d coordiante system?

stone cradle
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like this one

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with x y z

thin thicket
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die vector gleichung von der linie die durch A und B geht? sorry mein deutsch ist nicht so gut

stone cradle
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beim ersten bild wird das ganze ja mit einem 2d koordinatensystem gemacht also mit x und y.
ich vestehe aber nicht wie ich den vector AB bei x y z herausfinde, da man ja bei nur x y ablesen konnte

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ich verstehe nicht wie die das mit 7, -1, 2 herausgefunden haben

thin thicket
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der vector (7 | -1 | 2) ist B-A ( 4 - (-3) | 0 - (1) | 2 - 0)

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vector

stone cradle
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oh stimmt, das hatte ich sogar mal gemacht

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voll vergessen

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danke dir

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toxic tree
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toxic tree
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I dont get how they solved this one

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I understand that without the restriction its 54^5

thin thicket
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yes

toxic tree
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But idk how they got the 26^5

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to do 54^5 - 26^5

thin thicket
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we need the restriction and the 26^5 gets rid of the number of passwords with only lowercase

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cuz 54^5 includes the only lowercase and the mixed lower-upper

toxic tree
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Wait

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I do get it

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Cause its just the same again

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n = 26 and k = 5

thin thicket
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yes

toxic tree
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Gives all the only lower case ones

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Okokokok

thin thicket
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yep

toxic tree
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Got it

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Thanks!

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❤️

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thin thicket
#

np

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warm crater
#

Write the pseudocode for an algorithm that determines if a function is injective, assuming that you can iterate through all the elements in the domain and the co-domain in a finite number of steps.

warm crater
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Im in a Discrete math course following Kenneth Rosens Textbook for discrete math

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and im just wondering how should i refer to the co domain in pseudo code

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should i just reference it, like b1,...bm: corresponding elements of co-domain?

glossy zephyr
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Thats just up to your liking tbh given its pseudocode

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If we assume f: A -> B
A = {a1,a2...an}
B = {b1,b2...bm}
is intuitive enough

warm crater
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ok

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so could i technically also just refer to the output using f(a1)?

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in my algorithm

glossy zephyr
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Yes, but remember that would be the image of A instead

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Not the Codomain

warm crater
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ahh i see

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ok thank you

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manic helm
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manic helm
#

ive formed DEs $\frac{dx}{dz} = \frac{-(2x + z)}{x}, \frac{dy}{dz} = \frac{2y}{x}$ but stuck form there

soft zealotBOT
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ashyboi

glossy zephyr
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anyways, this is more like solving 1 diff. equation and 1 diff.system

manic helm
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i kinda just assumed the flow was steady state

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and there was no t dependence

glossy zephyr
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We basically want a function for the position, right?

manic helm
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yes

glossy zephyr
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Velocity is the derivative of position through time

glossy zephyr
# manic helm

So here we got this:

$\u = \frac{dR}{dt}=(4x+2z,-4y,-2x)$

soft zealotBOT
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∫ᴄ 𝐅·𝑑𝑟 = ∬ʀ ∇⨯𝐅 𝑑𝐴

glossy zephyr
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where R is the position

manic helm
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yes

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lemme give it a try

glossy zephyr
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Or, in another words

manic helm
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oh ye

glossy zephyr
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Here you can see that x and z depend on eachother

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and y depends on itself

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Soooo, they are basically
1 system
1 equation

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From solving those two separately, you can combine them back into vector notation

manic helm
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ye i see it

glossy zephyr
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tbh, a lot of physics problem are usually written down through dt

manic helm
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how come

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in here they dont do this

glossy zephyr
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They did