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1 messages · Page 990 of 1

loud oriole
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ok in the og pic, the number of terms is n-k, so why is the closed form n*c and not (n-1) * c

gray isle
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the number of terms isn't n-1

loud oriole
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oh i see then

gray isle
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just like how the number of integers from
1 to 100 (inclusive) isn't 100-1 = 99

loud oriole
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alright, then thx my g

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.close

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fossil mason
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How do I find qtp

lone heartBOT
fossil mason
gray isle
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what have you tried?

fossil mason
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I also tried looking for equilateral triangle

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Or like any dif side length

gray isle
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apply TQS is a sector of a circle with centre T

fossil mason
gray isle
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you're skipping ahead

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use what I mentioned to determine the length of QT

fossil mason
gray isle
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yes

lone heartBOT
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@fossil mason Has your question been resolved?

fossil mason
gray isle
#

determine your angle

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@fossil mason Has your question been resolved?

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gilded kiln
lone heartBOT
gilded kiln
#

How are they rewriting this

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doesnt 4/100 = .04 and 25/100 = .25, where are they getting the 5 and 2 squared for a and b

echo socket
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4/100 = 1/25 = 1/5^2

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25/100 = 1/4 = 1/2^2

gilded kiln
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ohhhh

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gilded kiln
#

$-4\sqrt{2}=81-b^2$

lone heartBOT
ocean sealBOT
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bababeeboo

gilded kiln
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subtracting 81 from both sides gets me -86.66, and then dividing both sides by -1 gets me 86.66, and then sqrting both sides gets me b = 9.31

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im finding b to be able to get the co-vertex for this question

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but the covertices (9.31), (-9.31) obv arent right since a has to be > b

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so im not sure what im doing wrong

lone heartBOT
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@gilded kiln Has your question been resolved?

gilded kiln
#

<@&286206848099549185>

ocean prism
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O wait

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Nvm

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Just read what u said

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Lemme a sec ill try the question

gilded kiln
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a calculator said the co-vertices are (7, 0) and (-7, 0) but I have no clue how they got that

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<@&286206848099549185> ?

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@gilded kiln Has your question been resolved?

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alpine sable
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A bit of an abstract question, but does anyone have any idea how I could normalise these results?

alpine sable
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For example, I would be able to say that if x_0 - x_c < 0 , the equilibrium is "skewed" toward the left, if it's > 0 towards the right and if it's = 0 then it's perfectly balanced

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but how could I quantify how much the skew really amounts to?

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Say, on a scale from 0 to 1 ?

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because x_0 - x_c is unbounded on both sides

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does x_0 - x_c / max(x_0, x_c) seem reasonable ?

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I'm unsure

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yea I think it does

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shell narwhal
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what does the c underbar math symbol means

shell narwhal
timid hazel
shell narwhal
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oh thanks

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.close

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tranquil hare
#

Can someone elucidate me on what I've done wrong here?

tranquil hare
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Retraced my steps but can't figure out how I'm screwing up at the signs.

buoyant kayak
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i'm not sure why you made the 14 negative when you had it positive on your last step

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also how'd you get 7 for your denominator

tranquil hare
tranquil hare
buoyant kayak
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are you sure the answer doesn't say -(x-14)

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oh wait that entire thing is over 7/(x^2-4)

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which shouldn't change the sign of the 14 but makes sense how you get 7 in the denominator lol

tranquil hare
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You're absolutely right

buoyant kayak
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alpine sable
lone heartBOT
limpid spade
#

t=3.4

naive valley
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is there a question somewhere?

limpid spade
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.26

alpine sable
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the refrence number

naive valley
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thank you for that information

limpid spade
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9 (2/2 Points) DETAILS PREVIOUS ANSWERS

alpine sable
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i subtracted 3.4 and got .26

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is that bad

naive valley
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very bad, possibly

alpine sable
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why it’s hw?

limpid spade
alpine sable
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💀💀

limpid spade
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we don't know

alpine sable
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i can show

limpid spade
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what the referemce

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number is

alpine sable
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see CatCry

alpine sable
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i saw a youtube video and it said to subtract pi

naive valley
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extraordinary

limpid spade
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mam

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mam

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man

naive valley
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i would definitely subtract pi in that case

limpid spade
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sum then all

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take the mean

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lineair it

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find the tangent line

naive valley
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the pi is a lie

limpid spade
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last chance

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show us

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the whole question

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part a, b and c

spark sky
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Lol

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That’s weird

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He doesn’t wanna @limpid spade

limpid spade
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why

spark sky
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I dunno

naive valley
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no one can answer the question if there is no question

spark sky
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Yeah for real

limpid spade
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t=3.4
.26
🧠

spark sky
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Amazing solution

naive valley
limpid spade
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Need Help?

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@alpine sable

alpine sable
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no sorry i just received it

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i posted it twice by accident

limpid spade
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?

naive valley
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extraordinary

limpid spade
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I'll just close it

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.close

lone heartBOT
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reef trout
lone heartBOT
reef trout
#

<@&286206848099549185>

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@reef trout Has your question been resolved?

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@reef trout Has your question been resolved?

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alpine sable
lone heartBOT
alpine sable
#

how do i find the integral from -3 to 0

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using only the graph?

umbral crest
alpine sable
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I'm aware

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but i don't know how to get that from a quarter circle

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wait

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so that area is 9pi/4, the quarter circle

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so... maybe subtract 9 (3*3) from that..? but thats a negative number

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ah

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9 - that number

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kek

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.close

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rich basin
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@bright hedge its because they lie on the unit circle

last ether
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@rich basin close your session lol

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viscid dew
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Why is dividing an amount by 0.5 = same as multiplying by 2 and vice versa (multiplying an amount by 0.5 the same as dividing by 2)?

last ether
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Yeah, good question

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So

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When you divide by half, you're asking the question "how many halves fit into this?"

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For example, when we do 1/0.5, how many halves go into 1?

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@viscid dew

viscid dew
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2 halfs exactly.

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Ah, it means the answer for 1/0.5 is 2.

last ether
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Yeah

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So how about this

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2/0.5

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How many halves go into 2?

viscid dew
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Four.

last ether
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Good

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How many halves go into 3?

viscid dew
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6 halfs.

last ether
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Ye

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So you'll notice that

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When we divide by 0.5, it results in doubling the dividend

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This is also explained by the fraction rule where:

$\frac{a}{\frac{b}{c}} = \frac{ac}{b}$

ocean sealBOT
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Umbraleviathan

viscid dew
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What about the opposite?

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"Multiplying an amount by 0.5 the same as dividing by 2".

last ether
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"What is half of this?"

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Multiplying by 0.5 is "what is half of this?"

Dividing by 0.5 is "how many halves fit into this?"

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Well

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Better question is

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What is 0.5 of this?

viscid dew
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I came up with this one: "how much [number] is fit in the sum"?

last ether
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Well

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Ehhhhh

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Sum?

viscid dew
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Because after all, the number could be anything.

viscid dew
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I'm partly using a translator because I'm not native English-speaker.

last ether
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Uh

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Sum is like

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Well

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In a colloquial context, I guess

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Although mathematically, not really

last ether
viscid dew
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Hmm, okay.

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Thanks you.

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.close

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ancient fjord
lone heartBOT
ancient fjord
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Hi, can someone help me with span?

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I didn't understand from where did the span {1,1} come from

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on the first example

verbal timber
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have you tried solving for x1 and x2 normally?

ancient fjord
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Yes

verbal timber
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what did you get

ancient fjord
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x1=1+x2

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x2=x1-1

verbal timber
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You should try solving it in row echelon form

ancient fjord
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But that is the result when I solve it with REF

verbal timber
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multiply the second row by 1/2

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then what do you notice about row 2 and row 1

ancient fjord
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Yes they are identical now

verbal timber
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So using gaussian elim. the second row is all zeros

ancient fjord
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yes

verbal timber
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leaving you with just x1-x2=1

ancient fjord
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x1 =1

verbal timber
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x1=1-x2

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x2 can be any number

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it is not necessarily 0

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so x2=k

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k is any number

ancient fjord
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okkk

verbal timber
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x1=1-k
x2=k

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write it in vector form

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(x1, x2)=(1, 0)+k(-1 1)

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if k can be any number then the solution includes the span of (1, 1)

ancient fjord
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aaa Okay I see, what if the solution is not any number

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The span would be the vectors

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that we found

lone heartBOT
#

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alpine sable
#

If I pick seven random digits and arrange them as a number, what's the chance the two last digits are equal? So from 0000000 to 9999999 (which has the last two numbers equal), what's the chance?

alpine sable
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I thought we could isolate the first five digits and think of the last two separately. There are 10 possible combinations for equal digits (00, 11, 22, etc), so I thought maybe one in each (10^5)*(10^2), which is one every 10^7, is that right?

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I forgot the properties on exponents, so I'm not sure if we add the exponent or multiply them.

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But multiplying is in nested exponents, so I thought my best bet would be adding them.

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Because for the 00 ending we have 10^5 possible numbers, for the 11 ending it's the same, all the way up to 99. So it's actually 10*(10^5), leaving me with 10^6 since 10=10^1, right?

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One in each 10^6 numbers have the last two digits equal.

atomic acorn
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you are right in your approach by ignoring the first 5 digits and making the last 2 same

atomic acorn
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the total numbers in your sample space is 0 to 9999999 which is 10^8 if I'm not wrong

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and the total numbers with last 2 digits same is 10^6 as calculated by yourself

alpine sable
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My objective is to find the probability that if I pick a random number with 7 digits, the last two are equal, as I've said. So it'd be fair to say that one every 10^6 seven digits numbers would have the last two digits equal, right?

atomic acorn
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Wait so would 0 be included then?

alpine sable
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Yep.

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0000000

atomic acorn
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coz that technically isn't 7 digits but ok

alpine sable
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Just fill it till it's 7

atomic acorn
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yeah got it

alpine sable
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12 would be 0000012

atomic acorn
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Your probability would be 10^6/10^8 right?

alpine sable
ocean sealBOT
alpine sable
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Yes

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$\frac{1}{100}$ is the probability then?

ocean sealBOT
atomic acorn
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Yea

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total cases / sample space right?

alpine sable
#

Heck yeah

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Can't thank ye guys enough.

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.close

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fallow pagoda
#

can someone help i don’t understand

atomic acorn
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What's your progress so far?

alpine sable
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Yeah that's some weird wording you've got there.

fallow pagoda
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pretty much nowhere

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i looked at the answer and k can’t get how they got it

alpine sable
atomic acorn
#

Ok so looking at a graph how can you determine if it's increasing decreasing?

fallow pagoda
#

up to right

alpine sable
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Generally just up will do.

fallow pagoda
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okay

atomic acorn
fallow pagoda
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so

atomic acorn
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You go from left to right in x

fallow pagoda
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yep ok

atomic acorn
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If the graph goes up its increasing

fallow pagoda
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okay and how do i find the domain doinf that

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because the answer doesn’t make sense to me

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(0, infinity)

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i dont see where the 0 came from

atomic acorn
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Before zero is the graph increasing or decreasing?

alpine sable
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Just in 0.

fallow pagoda
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ohh wait i think i see

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so for part ii) is is (0,-infinity)???

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because the answer says (-infinity,0)

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whys infinity in the x slot

alpine sable
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Nah, it's an interval, the greater number should be on the right.

fallow pagoda
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oh so it doesn’t matter about the x and y??

alpine sable
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Not sure I get what you're asking.

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It does matter.

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Question is: To which range of x, y increases if x increases?

fallow pagoda
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yeah no for part ii) i mean sorry

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it’s asking about the decreasing

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so before x= 0 the graph decreases from forever

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y being -infinity

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so isn’t it (0,-infinity)

alpine sable
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It's $(-\infty, 0)$.

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Wait, wrong.

ocean sealBOT
alpine sable
#

Now it's correct.

fallow pagoda
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yeah i’m confused why that is sorry

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why the infinity is at the front

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because that’s the y value is it not??

alpine sable
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Because it's lesser than 0.

fallow pagoda
#

or is infinity the x value when decreasing

alpine sable
#

Lesser on the left, greater on the right, that's how intervals work.

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If you invert the order you just get the null set.

alpine sable
lone heartBOT
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quiet lantern
#

Question regarding coefficient of determination within linear regression, does it simply mean how strong the linear regression in predicting future datapoints?

noble sinew
#

not future datapoints, no

hybrid zenith
#

It's more about proportion of change in 1 that can be predicted by the other varibale
As opposed to extrapolation
As far as I remember

hybrid zenith
#

The slope would surely just be change in y/change in x
This is to do with how much variation in a variable is because of the change in the other

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I'm not rlly sure how best to word it sorry

oblique spire
noble sinew
#

no it means the R^2 value

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and as I said it doesn't mean strength of predicting future datapoints

oblique spire
hybrid zenith
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They said coefficient of determination =r^2

oblique spire
#

Oh

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I see

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I thought it's the slope

quiet lantern
noble sinew
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how much of the variation in respone is explained by the explanatory variables

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is one way to view it

oblique spire
#

R^2 does indicate the performance of the model ig

quiet lantern
oblique spire
#

R2 is a measure of the goodness of fit of a model. In regression, the R2 coefficient of determination is a statistical measure of how well the regression predictions approximate the real data points. An R2 of 1 indicates that the regression predictions perfectly fit the data.

It's written directly on wiki ig

noble sinew
#

magnet wtf

quiet lantern
#

yeah its supposedly an analogy to explain it better to me haha

oblique spire
#

Lol like the line is a magnet attraticing the data points

quiet lantern
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yeah

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so like given that its 0.314, thats considered "weak" relatively

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but if it was say 0.7 that would be considered 70% powerful

oblique spire
#

From what I learned in machine learning

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Ya

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R2 is indeed the performance of a model

quiet lantern
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and 100% would mean that all datapoints is right on the line itself

hybrid zenith
#

Ig the magnet analogy works
Tho u just need to make it clear in the exam that its to do with variables
As opposed to the line being a magnet lol

oblique spire
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Dude can I just ask if I understand the question correctly did u meant if r^2 is the measure of how good the line of best fit is

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Or how well it will perform on unseen data

quiet lantern
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Can it be used as a predictor for future datapoints?

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Or is it completely useless in that?

oblique spire
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No?

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R2 is a evaluator rather

#

It tells us

#

How good the model is

quiet lantern
#

The model being the line itself, yeah?

oblique spire
#

Ya

#

R2 tells the marks received by the line

quiet lantern
#

And the line itself says "in a perfect case scenario, the datapoint should be on this line"

#

And that "perfect case scenario" is represented with R2 at 1.0 (or 100%)

oblique spire
#

Ya

#

The higher r2 the better

#

Ig

quiet lantern
#

yeah

oblique spire
# quiet lantern yeah

So summing it all up R2 atleast in the field where I worked in is used more of a metric to evaluate the performance of a linear regression model

quiet lantern
#

So I'm asking that because another friend was arguing against the person that explained the analogy... And said that 0.5 R2 means that there is no relation

#

so I got confused 😂

#

I'm receiving two opposite explanation and didn't know what to believe

noble sinew
#

you can't just only look at R^2 and say higher R^2 is better

#

very false

oblique spire
#

I don't think so

quiet lantern
oblique spire
#

R2 is a measure of the goodness of fit of a model. In regression, the R2 coefficient of determination is a statistical measure of how well the regression predictions approximate the real data points. An R2 of 1 indicates that the regression predictions perfectly fit the data

#

It's written on wikipedia ig

#

Unless u meant something called overfitting

oblique spire
#

We still have stuff like RSS

#

They are different metrics

noble sinew
#

just saying

oblique spire
#

I can't even read the meme

#

Oof

quiet lantern
#

yeah quality is a bit weak hard to read numbers

noble sinew
#

numbers are useless

#

just look at the plots

oblique spire
#

Did

noble sinew
#

so clearly R^2 doesn't say anything about how good your model is in itself

oblique spire
#

It is saying in one way

#

U have to generalize

#

Between other metrics

#

LIKE rss

#

Such that it works well on almost all

#

It's just one of the many metrics

quiet lantern
noble sinew
#

wdym work?

quiet lantern
#

as in the model is "predicting" the datapoints

oblique spire
#

Well I have seen machine learning engineers use r2 to evaluate linear regression models

#

All the time

#

Maybe it's applications are a bit different in normal stats

noble sinew
#

You can fit a linear regression with a R^2 very low just fine

#

as long as assumptions are met

oblique spire
#

No?

quiet lantern
#

Ah

noble sinew
#

all this means is your confidence intervals will be bigger

#

yes you can

#

the assumption that variance is the same at all points and they are independent

#

only asusmption needed for linear regression

quiet lantern
#

I think what @noble sinew means is that the data has to be:
1- homoskedastic
2- linear
3- independent
4- normality in observations

#

is that it?

oblique spire
#

I mean a good model can have a bad R^2 but a model with good R^2 will generally be a good model

noble sinew
#

yes

oblique spire
#

That doesn't make my statement

#

Wrong

#

Ig

#

That high r^2 is good

#

But low r^2 isn't bad either

noble sinew
#

you said the higher the better

oblique spire
#

Oh right mb

quiet lantern
#

In most situations would it still be false?

#

that the higher is better?

noble sinew
#

step 1 before fitting linear regression is plotting your data (assuming the dimensions actually allow you to look at your data)

#

but assuming assumptions met then all higher R^2 does is make your length of conf interval shorter

#

that is all it does

quiet lantern
#

okay one more question, i know that R2 is calculated using different formulas and I've seen many websites calculate R2 by simply multiply correlation by itself...

Was just wondering if R2 can always be calculated by simply multiplying correlation by itself? Because that seems like a very simple formula to calculate something that is supposedly complex with a lot of assumptions in it...

noble sinew
#

if you mean Pearsons correlation coefficient then yes

quiet lantern
#

Isn't R2 coefficient of determination? 🤔

noble sinew
#

which is persons correlation coefficient squared?

quiet lantern
#

That is what I'm asking

#

Can it always be calculated by multiplying correlation by itself?

quiet lantern
#

Oh, there are other types of correlation? 😅

#

(do they give different answers too?)

noble sinew
#

yes there are many (pearsons is the standard one)

quiet lantern
#

So if I want to calculate R2 I can only use pearson

#

but if I used another correlation?

#

It would give a wrong R2 ?

#

I always thought correlation is -1 to 1 and assumed that it is always the same number

quiet lantern
noble sinew
#

Because they are defined differently yes ofc you get another number

quiet lantern
#

Thank you for your help everyone @noble sinew @oblique spire @hybrid zenith

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strong hornet
#

.. don't ping helpers right away

lone heartBOT
#

@thick lynx Has your question been resolved?

noble sinew
#

Depends on the questions. Typically competition math doesn’t have a lot to do with standard syllabus

#

Depends on the questions you call hard

velvet pelican
#

hard school maths is not really comparable to competition math

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delicate widget
#

I’ve gotten 48

lone heartBOT
delicate widget
#

Wrong question

lone heartBOT
#
Channel closed

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delicate widget
#

Help with c

#

ive gotten 48

#

but since 48 is in the group 200<w<400 i dont know how to deal with it

#

its in between the 3rd and 4th group

#

im getting confused

#

how to comment on if hes correct

#

ping me if anyone comes to help me

lone heartBOT
#
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delicate widget
#

d

lone heartBOT
delicate widget
#

randomlyly closed

delicate widget
#

<@&286206848099549185>

#

noone

#

sad

#

.close

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uneven wing
#

The Exercise: ABC a non-isosceles right triangle at A. I is the midpoint of [BC] and [AH] is its height from A. Show that: cos IAH = 2AB*AC/BC²

I didn't understand how to show it , I consider your help thank you

open folio
#

Have you drawn it out?

uneven wing
#

lemme do it real quick

#

it look like this

#

hmmm breziboi?

#

<@&286206848099549185>

oblique spire
#

IAH

#

Ig

uneven wing
#

can you help me?

oblique spire
lone heartBOT
#

@uneven wing Has your question been resolved?

lone heartBOT
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frigid verge
lone heartBOT
frigid verge
#

I just want to check I did it right... in the blue pen

lone heartBOT
#

@frigid verge Has your question been resolved?

frigid verge
#

<@&286206848099549185>

#

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alpine sable
#

Hello , im having trouble understanding shanon entropy , i need to find out for example how the entropy is for a password consisting of 4 numbers ( 0-9 so 10 posibilities)
So i calculated -sum k=1 to 2 ( 1/10)*log2(1/10) but the answer seems wrong

alpine sable
#

Found the solution the sum was wrong

#

.close

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native kite
#

so this problem is assigned to me by my teacher

native kite
#

and I answered with this, but I'm not confident with my answer, so I would like to ask anyone if this is correct?

#

I'm asking because I have very little knowledge on limits and continuities and I just watched some videos about it and tried to answer based on my understanding

last ether
#

Okay so you have the right idea, but not quite

#

When checking for continuity within [-1, 1]

#

Check to see if the area to the left of -1 would be equal to the area right of -1

#

Aka, check a = -1

#

And a = 1

native kite
#

ohh okay got it

#

so is 1 and 3 correct?

lone heartBOT
#

@native kite Has your question been resolved?

native kite
#

<@&286206848099549185>

uncut cobalt
#

help

native kite
harsh swallow
#

,rotate

ocean sealBOT
native kite
#

so I tried something like this, both a=1 and a=-1 aren't equal

#

so it's discontinuous?

harsh swallow
#

it's not continuous on the interval [-1,1]

#

which you have shown

#

by showing that the function has a jump

#

from -8 to -3

native kite
#

okay got it

native kite
harsh swallow
#

i'm guessing what they mean is at x = 1

#

if it is continuous at x = 1

native kite
harsh swallow
#

meaning the limits from the left and the right are the same and equal to the value at that point

#

from the left you used the wrong part

#

you used the first part of the function but you should've used the second part

#

because we're looking at x = 1

native kite
#

oh I just noticed that

#

okay thank you very much

harsh swallow
#

also 3 isn't correct if i remember correctly

native kite
#

how do I do that?

harsh swallow
#

the reasoning as to why it is discontinuous is incorrect

native kite
#

okay got it

#

I'll figure the rest on my own

#

thank you very much!

#

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slender girder
lone heartBOT
slender girder
#

How do you approach this

rotund plank
#

Then discriminat

slender girder
#

Done

rotund plank
#

Assume x ≠ 1

slender girder
#

I simplified it

#

lett me show you

#

where i got to

#

x^2-2x(m^1/2)+2(m)^1/2((m)^(1/2) + 1)) -3 = 0

#

since it has no real roots

#

but how do i do the discriminant from here onforth

#

whats the a, b , c

slender girder
#

wait what

rotund plank
#

It’s equal to zero

slender girder
#

it has no real roots

#

ohh

rotund plank
#

You’ve confused the equation with the discriminant

slender girder
#

the discriminantt is less than 0

#

my bad

#

yes yes

rotund plank
#

Just get coefficients of x^2, x, and a constant term

#

Those are your ‘a’, ‘b’, and ‘c’

slender girder
#

the coefficient of x is 2 ?

#

and x^2 is 1 ?

rotund plank
#

No

slender girder
#

oof

#

how

rotund plank
#

What happens to sqrt(m)

#

You forgot about it

slender girder
#

is it 2sqrtm?

rotund plank
#

Nope

slender girder
#

hmm

rotund plank
#

ax^2 + bx + c = 0

slender girder
#

im a bit lost as you can see

#

c would be -3 right?

rotund plank
#

There’s a minus in front of that term

#

No

slender girder
#

what

rotund plank
#

2sqrt(m)(sqrt(m)+1) is a constant

slender girder
#

okay

#

so thats c

#

what happened to the -3 then?

rotund plank
#

It’s also part of c

#

That whole end bit is c

slender girder
#

2sqrt(m)(sqrt(m)+1) - 3 ?

rotund plank
#

Yep

slender girder
#

right

#

so b would be 2sqrt(m)

rotund plank
#

-2sqrt(m)

slender girder
#

ahh

#

and a would be 1

rotund plank
#

Yea

slender girder
#

oo

#

(-2sqrt(m)^2-4(1)(2sqrt(m))(sqrt(m)+1)-3))<0

#

shouldnt m be greater than or equal to 0

#

im actually lost

#

@rotund plank sorry for the tag

#

could you elaborate on how i'd go about solving it now

rotund plank
#

Yes it should

#

Just expand it

lone heartBOT
#

@slender girder Has your question been resolved?

slender girder
#

I did

#

I got -10m -8sqrtm I think

#

-10 m - 8 sqrt(m) - 3<0

#

how ?

#

<@&286206848099549185>

slender girder
#

.close

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fossil valley
lone heartBOT
fossil valley
#

I don’t even know where to start with these two questions

#

dP/dt = Kt?

#

P=Ce^(kt)

#

For the first one

#

Is that correct

fervent timber
fossil valley
#

Yeah now I do

#

Had to remember 💀

#

Now for the other one

#

I am stuck

lone heartBOT
#

@fossil valley Has your question been resolved?

fervent timber
lone heartBOT
#

@fossil valley Has your question been resolved?

lone heartBOT
#
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thorn vale
#

The speed of sound at sea level is 760 mph (mach 1.0) what is the speed in mph of a bullet traveling at mach 0.7?

I subtracted 0.7 from 1.0 and got a 0.3 difference. Do I then divide 760 by 3?

marsh rapids
#

mach 0.7 means 0.7 times the speed of sound

#

no need to overcomplicate this

thorn vale
#

Ok

#

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slender girder
lone heartBOT
slender girder
#

I worked out A

#

How would I approach b and c

oak chasm
#

For b, get the second, fourth, and eighth terms of the arithmetic sequence.

#

Use those as the second, third and fourth terms of the affine sequence.

#

Use the formula and fill in values you know.

#

You should get two equations from that.

#

And you have two variables to solve for.

#

@slender girder

slender girder
#

I tried

#

To do that

#

so u2 =8

#

u4 = 18

#

u8 = 38

#

in the original arithmetic sequece

#

they're telling us u2= ru_3 + s and u4= ru_2 + s ?

oak chasm
#

So, in the affine sequence,

u₂ = 8
u₃ = 18
u₄ = 38

u₃ = ru₂ + s

#

u₄ = ru₃ + s

#

You got the indexes mixed up.

#

The larger one is on the left of =.

slender girder
#

oh

#

so 18=ru_2 + s ?

#

and 38=ru_3 + s

oak chasm
#

Yes, and fill in u₂ and u₃ there.

slender girder
#

Ohhh

#

we use our initial values ?

oak chasm
#

18 = 8r + s
38 = 18r + s

slender girder
#

so like u2 is 8

#

OHHH

#

im an idiot

oak chasm
#

Now it's just simultaneous equations.

slender girder
#

we can just subtract

#

and solve for r first

#

no ?

oak chasm
#

Yeah r in terms of s.

slender girder
#

r=2

oak chasm
#

Oh, I see.

#

Yes, that works.

slender girder
#

s=2

#

does that make sense

#

for both of them to be the same valuie

oak chasm
#

Yes, as long as it works.

#

Check with the original equations.

slender girder
#

So i found r to be 2

#

so I did

oak chasm
#

18 = 8r + s
38 = 18r + s

slender girder
#

38=18(2) + s

#

38-36 = s

#

s = 2

oak chasm
#

18 = 8(2) + (2)
38 = 18(2) + (2)

#

If those are truly equal, you got the right answer.

#

For C, I'd guess you use induction.

#

On m.

slender girder
#

how

#

would you use induction

oak chasm
#

Well, you start with m = 1.

slender girder
#

oh wait

#

they rewrote the q

oak chasm
#

You show that the formula given in part C is equal to the equivalent from just before B.

slender girder
#

lemme send you the reworded q

oak chasm
#

OK.

slender girder
#

Show that the mth term of the affine sequence equals the 2^(m-1)th term of the arithmetic sequence.

oak chasm
#

That's C part ii.

slender girder
#

ii ^

#

yeah

oak chasm
#

For C part i, induction.

slender girder
#

proof by induction ?

oak chasm
#

Yes.

slender girder
#

so we plug in any two values

#

of r and s

#

that are the same ?

oak chasm
#

uₘ₊₁ = ruₘ + s
uₘ₊₁ = rᵐu₁ + s((rᵐ - 1)/(r - 1))

#

No, you leave r and s as variables.

#

It's asking you to show it for all r and s.

#

So, r and s can't take on any particular value when you're proving it, or else you've only proven it for those values.

slender girder
#

ohh

#

alr

#

do we use a particular value of u1 etc ?

oak chasm
#

No, but start with m = 1.

slender girder
#

if m=1

#

itll be u1

oak chasm
#

What will be u₁?

slender girder
#

u_(m+1)

oak chasm
#

No, m = 1 means m + 1 = 2.

slender girder
#

oh itll be u_2

oak chasm
#

But we don't exactly need that.

slender girder
#

okay so u_2=r*u_2 + s

oak chasm
#

uₘ₊₁ = ruₘ + s
uₘ₊₁ = rᵐu₁ + s((rᵐ - 1)/(r - 1))

So,
ruₘ + s = rᵐu₁ + s((rᵐ - 1)/(r - 1))

slender girder
#

okay sure

oak chasm
#

So, prove that for m = 1.

slender girder
#

They're both u_m+1

oak chasm
#

Right, so they're both equal to each other.

slender girder
#

yes

#

so its like saying

#

1=1

oak chasm
#

No, it's not.

slender girder
#

why

#

isnt it

oak chasm
#

The form of the thing on the left and right aren't the same.

slender girder
#

you want me to manipulate it?

oak chasm
#

It's not immediately apparent that they're actually equal.

#

You need to prove that, unlike with 1 = 1.

slender girder
#

Ohh okay

#

so should i work out both values at m = 1

oak chasm
#

Yes.

slender girder
#

and show that they're equal?

oak chasm
#

Right.

slender girder
#

I mean

#

I got to

#

U_2=r^1 * u_1 + s(r^1-1)/(r-1)

#

dont they cancel

#

the r-1/r-1

oak chasm
#

No, m = 1, not 2.

slender girder
#

yes

#

but its U_m+1

#

1 + 1 = 2

#

so it becomes u_2

oak chasm
#

Yes, (r - 1)/(r - 1) cancels.

#

No.

#

They give you two formulas for the sequence.

#

One is known to be correct.

#

The one above part B.

#

It's a recursive formula.

#

One isn't.

#

It's a more direct formula.

slender girder
#

oh

#

is the correct one the second

oak chasm
#

No, it's the formula above question B.

slender girder
#

Oh

oak chasm
#

The recursive formula.

#

In C part i, they give you the closed form version.

#

Where you don't have to go through each and every previous item in the sequence like with the recursive version.

slender girder
#

right

oak chasm
#

The recursive formula is correct, but the closed formula might not be.

#

So, you set the formulas equal to each other and then you use induction to show that they are indeed equal for any m.

slender girder
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the thing is

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if we used 1

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in the closed formula

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we'd get u_2 = r * u1 + s

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after cancelling the r-1's

oak chasm
#

Right, but set the formulas equal to each other.

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Not equal to uₘ₊₁.

slender girder
#

ohhh

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r*u_m + s = r * u_m + s

oak chasm
#

Well, fill in the ms.

slender girder
#

yeah

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1

oak chasm
#

So, the formulas are the same for m = 1.

slender girder
#

r*u1 + s = r x u1 + s

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yes

oak chasm
#

That's the base case.

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Now we do the inductive case.

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Let's say it works for a certain m. Does it then definitely work for m + 1?

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ruₘ + s = rᵐu₁ + s((rᵐ - 1)/(r - 1))

slender girder
#

I suppose so?

oak chasm
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We want to prove that.

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And the inductive hypothesis is that it worked for the previous m.

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ruₘ₋₁ + s = rᵐ⁻¹ u₁ + s((rᵐ⁻¹ - 1)/(r - 1))

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The previous m worked, so we know for sure that that equation is correct.

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How can we transform that equation into ruₘ + s = rᵐu₁ + s((rᵐ - 1)/(r - 1))?

slender girder
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m-1 ?

oak chasm
#

Yes, with induction, you have the base case, and we did that with m = 1.

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You also have the inductive case, where you assume it worked for the previous number and show it works for this number.

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The assumption is called the inductive hypothesis.

slender girder
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Oh I see

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Wait could I ask

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how old are you

oak chasm
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Inductive hypothesis:

ruₘ₋₁ + s = rᵐ⁻¹ u₁ + s((rᵐ⁻¹ - 1)/(r - 1))

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Oh, an adult, but I don't want to get more specific than that.

slender girder
#

Ahh I see

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No worries

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if we set m to 2 ?

oak chasm
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No.

slender girder
#

it would look the same as the one above

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2-1 = 1 ?

oak chasm
#

The base case has the a particular value for m.

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The inductive case doesn't.

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Here's how induction works.

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You show it works for one number.

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You show that if it works for a number, it works for the next number.

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So, if it works for m = 1, then it will work for m = 2.

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Then, if it works for m = 2, it will work for m = 3.

slender girder
#

we want to know if it works when m = 0 now ?

oak chasm
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And so on.

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No.

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Do you understand how induction works?

slender girder
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yes

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we prove that a statement holds true

oak chasm
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How does it work?

slender girder
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for every natural number

oak chasm
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No.

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With induction, we prove that it works for one number. It could be 53.

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Then, we prove that if it works for one number, it works for the next.

slender girder
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The inductive step in a proof by induction is to show that for any choice of k, if P(k) is true, then P(k+1) is true.

oak chasm
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Yes, that's what I said.

slender girder
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So we showed that P(k) is true

oak chasm
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No.

slender girder
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bruh

oak chasm
#

That is definitely not it.

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We assume P(k) worked.

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We don't prove it.

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We assume it without proof.

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Let's forget the mathematical notation for a bit.

slender girder
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Could we go onto another question, and get back to this one

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in a little while?

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a much easier question

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I simplified this

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as much as I could

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Now I'm struggling to find the values of m that satisfy the equation if it has no roots

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I knew that the discriminant must be <0

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i.e. b^2-4ac <0

oak chasm
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Please show your work.

slender girder
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One sec

oak chasm
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You didn't distribute the 4 to the - 3.

slender girder
#

oh -12

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not -3 ?

oak chasm
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Yes, there are other errors there, too.

slender girder
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Oh really

oak chasm
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(-2 sqrt(m))² is not -10 m.

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8 sqrt(m)(sqrt(m) + 1) is not 8 sqrt(m).

slender girder
#

its -4m

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or

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4m

oak chasm
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Right.

slender girder
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8m + 8 ?

slender girder
oak chasm
#

Just distribute.

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8 sqrt(m)(sqrt(m) + 1)

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8 sqrt(m) sqrt(m) + 8 sqrt(m)

slender girder
#

-8sqrt(m) * sqrt(m) + 1 would be -8m - 8sqrtm

oak chasm
#

Right.

slender girder
#

so

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-4m - 8sqrtm - 12 <0

oak chasm
#

How did you get -4m?

slender girder
#

oh

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double negative ?

oak chasm
#

Ahh, OK.

slender girder
#

so 12m?

oak chasm
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No, it's 4m for the b²

slender girder
#

yes

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so 4m-(-8m-8sqrt(m)-12) < 0 ?

oak chasm
#

So, 4m - 8m - 8 sqrt(m) + 12

slender girder
#

okay

oak chasm
#

-4m - 8 sqrt(m) + 12

slender girder
#

yup

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so how would we go about finding m nopw

oak chasm
#

OK, so:

-4m - 8 sqrt(m) + 12 < 0
m + 2 sqrt(m) - 3 > 0

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Now, we can take sqrt(m) as the value we find via quadratic formula.

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a = 1, b = 2, c = -3 will give us what sqrt(m) can be at the boundary between where -4m - 8 sqrt(m) + 12 goes from greater than or equal to 0 to being less than 0.

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Does that make sense?

slender girder
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yeah

oak chasm
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You can also do a = -4, b = -8, c = 12.

slender girder
#

now we plug those into the quadratic formula

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x=1, x = -3

oak chasm
#

Right, and sqrt(m) can't be negative.

slender girder
#

yeah

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so m => 1 ?

oak chasm
#

Now, since the x² coefficient is positive, we know it points up.

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The U of the quadratic does.

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So, > 0 is on the outside of the roots.

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m ≥ 1 is almost correct.

slender girder
#

oh

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1/2 ?

oak chasm
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It's sqrt(m) and >.

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> because we don't want when it equals 0.

slender girder
#

ohh

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sqrt(m)>1 ?

oak chasm
#

Right.

slender girder
#

so m>1 ?

oak chasm
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Yes.

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Let's check that.

slender girder
#

yeah lets

oak chasm
#

-4m - 8 sqrt(m) + 12

slender girder
#

x^2-1/x-1

oak chasm
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-4(1) - 8 sqrt(1) + 12 = -4 - 8 + 12 = 0.

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So, that's a good sign.

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The discriminant is 0 exactly at m = 1.

slender girder
#

that means it has a root though

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we want it to be less than 0

oak chasm
#

Sure, but we have m > 1, which doesn't include m = 1.

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However, we want to check m = 1 to make sure it's 0.

slender girder
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Ahh

oak chasm
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The discriminant is continuous, so when it goes negative, it was just at 0.

slender girder
#

Makes sense

oak chasm
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Then we try with m = 2.

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-4(2) - 8 sqrt(2) + 12 = 8 - 8 sqrt(2) < 0.

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So, that's good.

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And that's all we really need to check.

slender girder
#

Perfect

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Thanks so much!

oak chasm
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No problem.

slender girder
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You're really well versed in maths

oak chasm
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Well, at the calculus and below parts.

slender girder
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You like calculus ?

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Oh lord

oak chasm
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No, it's not that I like it, it's just about as far as I remember from school.

slender girder
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Ahh

oak chasm
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I remember some linear algebra as well and some abstract algebra.

slender girder
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Do you remember distributions?

oak chasm
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Not really.

slender girder
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Like a question like this

oak chasm
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No, it's been too long.

slender girder
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Ahh I see

oak chasm
#

Let's go over induction right quick.

slender girder
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Yeah

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Lets

oak chasm
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You prove it works for a number, like 53.

slender girder
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which we did

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like 1

oak chasm
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Then you prove that if it works for one number, it works for the next.

slender girder
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No we didnt

oak chasm
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You don't prove that it works for that number.

slender girder
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We assume it does

oak chasm
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You assume that it works for that number without proof.