#help-26
1 messages · Page 52 of 1
it's canadian question
i mean canada math is just lame
also what's derivative? I think this belongs in calc right?
if they get a parabola by the substitution they could find the vertex of A(x) or A(y)
Basically the rate of change of a function
Oh yeah
yup never learned that yet
sorry im so lost rn


You only need one
ok
It’s a negative parabola so if you find the vertex that’s the maximum
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competition math
anyone want to help?
i can start with the fact that with a quadruplet of 4 points on the octagon, there is one pair of diagonals with an internal intersection
so there are 70 internal intersections, but are not distinct
i want to remove all the double/mutiple counts
obviously the center is one
there are 6 possibilities to remove
so i am thinking it is 49 just from the fact that we alr have at most 64 as the answer
but im not sure
ok no yes 49 is correct but i need to know how
wait i just realized that doesnt work 😅
so i just guessed it correct
wait no im confused
there are 4 directly opposite diagonals
4 choose 2 yields 6 possible diagonal intersections that we overcount
so we must subtract 5
to get 65
now we have to show that there exists at least one double count to arrive at 49
this would mean that we have three concurrent lines
other than the longest diagonal
the diagonal cannot skip over only one point
beacuse then you cannot have a triplet of concurrent lines
well you did guess it
but that's kinda how I would approach it
what happens if there was another option?
we have 4 diagonals intersecting at the center right
like 52?
yes
that gives 6 intersection points that we count as the same
so we subtract 5
you understand how i got 70 for the total right?
there will be more double counts
and
there is only one option that's less than our number, which is 65
being 40
49*
49 mb
but how do you know there are more double counts
i drew a picture
drawing is the most clear option
obviously one of the three diagonals cannot skip over only one point, right?
and we cannot have two longest diagonals right?
the longest diagonal being the one that goes directly oppostie
when i say that a diagonal skips over a point i just mean that it connects two points with a single point between them
in the picture above the diagonals skip two points
if we have two D_3 (diagonal that skips three points) then we intersect in the middle whihc we alr counted
and if we have 0 D_3 then we must have 3 D_2 but as pictured above that doesnt work
so then there is 1 D_3 and 2 D_2
wait
OHHHH IM SO STUPID
lol i was trying to draw it up for you
i can just reflect one D_2 diagonal over a D_3 diagonal to yield another D_2 diagonal that together forms a triple of concurrent lnies
either consec D_2's or not
so we do have more double counts
but how do i get to 49
well lets see there are 4 potential lines of symmetry
and then there are two possibilites for each of them
so i get 8 triples
each triplet has three intersections
i subtract two as to now overcount
so i do
65 - 8 * 2
which is 49
yayyy
woohoo
i did it
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thanks for your help @strong citrus
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need someone to check my work on related rates
@hardy steppe Has your question been resolved?
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Hello can somebody help me solve this, or walk me through it
have you encountered the quadratic formula
yes but do you know nay other ways i can solve it
you wont be able to factor it in any other way, as ||it has no solutions||
I had a problem on my quiz with irrational coefficients and use the quadratic formula on it when we had 3 minutes per problem and ended up getting √5000 something. (I failed the quiz)
💀
you can do the discriminant test to determine it has no roots though
No real solutions*
if you do the full quadratic formula you just end up with imaginary solutions
true, but I assume someone asking this question isn't at a level of math such that they are expected to find imaginary solutions
oh ok thanks
regardless, note that $\sqrt{b^2-4ac} < 0$ and therefore there are no real* solutions
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help🥲
just need guide through question
so the first thing first, what is the period of sin(x)?
is it 8pi
well I meant in general, but yes for your fucntion it's 8 pi lol
o
what is the period of a normal sine function?
is it 2pi
bingo
so we need to see how we translate for 2pi to 8pi
we can do that with the following: $2 \pi = x8 \pi$. Solve for x. That'll give you your scalar for your sin function
no wait
4?
MellowDramaLlama
sorry I had it backward lmao
yeah so solve for x just like you did before
1/4?
bingo!
hey would you look at that? 😮
since you didn't have to consider vertical stretch, vertical translation, or horizontal translation, you're good to go 🙂
is it 3
ohh okay this isnt that bad
where it gets trickier is when you have vert/hor translations, vert/hor stretches all at the same time haha
lemme do this rq and see if i need help again
sounds good
i understand the period of this is 2pi
right
this wavelength is built a lil different tho
it doesnt go through the middle perfectly like the other
yeah so this one is a horizontal translation
ummmm
I guess you can take your best guess? I hate when they do that to you though
it looks like y = 0.75, would you agree?
watchu mean
it's not crossing at y = 1, it looks a little bit less
but not as small as y = 1/2
so it's somehwere between that
best guess I would say is that it's crosses the y axis at y = 0.75
(or 3/4)
are u talking about like here when it first goes through
I'm talk about where it crosses the y axis, aka the horizontal translation
yeah exactly
I'm arguing that since it's not a nice number and we can't see the value we have to guess
which sucks
I hate when they make you do that
buuut I'm saying it's not 1, it's not 1/2, it's somewehre in between
yeah i gotchu now
so I was saying my best guess would be 3/4
so im trying to transform this to the like orginal way sine is?
omg I"m in an idiot I"m so sorry
I was looking at the graph like a vertical translation 😦
horizontal translation would be somthing like y = sin(x - d)
so we have to find d
how i do that
where does sin(x) normally cross the x axis?
0?
pi/2?
nope
pi
so we can see in your graph that it crosses the x axis at 3pi/4, which means we need to translate the graph over pi/4 units to the left
so how can we do that?
-pi/2?
no too much
we only need to shift by pi/4
ok let's take something more simple
something like y = 2x + 4
how would we shift it 1 unit to the right?
-1 where though in the equation?
oh ok
every function, no matter what, will abide by these properties
so if you want to shift something to the left, you add a value into where x is (aka within the parentheses). If you want to shift something to the right, you subtract a value similarly
so we're shifting to the left by pi/4 units, so what should we do?
we have y = sin(x - h) here
Here's another chart if it helps
in this case to the left
it's crossing the x axis at x = 3pi/4, which is less than pi
so we going this way?
yes correct
so its gone be sin(x+d)
pi/2?
nope
normally it crosses the x axis over at pi, we're not crossing it over by 3pi/4,
so what's pi - 3pi/4?
bingo
oh i said that before i edited it
so what's the equation? 🙂
there's a general rule that `sin(x) = cos(x - pi/2)
so instead you'll just do cos(x - pi/2 + pi/4) and simplify
3pi/4?
postiive or negative?
postive?
,w pi/2 - pi/4
it's positive pi/4
double check your work
did you just guess or did you work that out?
i thought we were adding pi/2+pi/4
it's -pi/2 + pi/4
postive?
there ya go, that's how to find the cos equiv 🙂
i got negative pi/4
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taking ap calculus and this is supposed to be relatively easy but i’m not sure on what to do after i take the derative of the original problem😭
do you recall what happens when we have a horizontal tangent? What does the slope equal?
slope = derivative
that’s all i know
and OHH
DO WE EQUAL IT TO 0?
we dooo right
but idk what after lol😭🙈
you're on the right track
so let's take problem 12
so we set f'(x) = 0
so what do we have there?
well you can, it's just 0
yeah
okayy hmmmm
like literally set f'(x) = 0 = 2x^3 + 3x^2 + x
now we have to solve for x
so we have this:
0 = 2x^3 + 3x^2 + x
what's something we can do to simplify this?
factor?
eve:P
eve:P
nvm that wouldn’t work cuz of the one
This is correct
Finally, 2x^2 + 3x + 1 looks pretty factorable to me
so far we have 0 = x(2x^2 + 3x + 1)
coolio, waht did you get?
(2x+1)(x+1)
there's one more 🙂
what about this little fella?
UHHHHH x= 0?
bingo
dangggt
oh hey look at that
that was it and i was stressing lol
fire
ooooo i didn’t even know that could happen
exactly what we expected
the way i alr got one horizontal tangent randomly 😭
just not in the right way cuz i did twice the derivative .. but hey i still got x= -1/2
well what you found there was an inflection point
that's the 2nd derivative
well
oh that's the 3rd derivative lol
omg it was
what was i even doing i couldn’t even fully do the first derivative
😭😭🙈
anywaysss thank u sm for ur help :)
yeah no problem. Also what would help is notation:
f'(x) = 1st derivative
f''(x) = 2nd derivative
f'''(x) = 3rd derivative
write the same number of ticks for which derivative you're taking
how far does it go Omg😭😭 i’m scared for when that comes up later
anything past the 4th derivative you just write the number in parentheses
ahhh
for example like if you had the 5th derivative it'd be f^(5)
Her'es a simple example
$f(x) = x^6\\f'(x) = 6x^5\\f''(x)= 30x^4\\f'''(x) = 120x^3\\f''''(x) = 360x^2\\ f^{(5)} = 720x \\ f^{(6)} = 720$
MellowDramaLlama
after about 3 to 4 ticks you can switch notation
you can technically use the (a) notation for any of them, but by convention the ticks are easier
well when you get into oscillating functions like sin and cos it gets crazy
for example
so we know the following facts:
$f(x) = sin(x)\f'(x) = cos(x)\f''(x) = -sin(x)\f'''(x) = -cos(x)\f^{(4)} = sin(x)$
MellowDramaLlama
make sense, right?
what would the 5th derivative be? At the 4th derivative we come back to the original function
so it would be
ah ok
$f^(5) = cos (x)$
eve:P
Bingo!
off the top of your head, how would you know something like the 385th derivative of sin(x)?
ummmm i wouldn’t know😭 maybe knowing it’s divisible by 5 it wojld make it cos (x) ?? idk
oooh you are so close
it correlates to the remainder when you divide the derivative number and divide it by 4
so what is the remainder of 385/4?
96.25
no sorry just the remainder
not quite, remember a reaminder has to be a either a postive integer or a 0
OHHH
it does correlate to .25 however 🙂
it’s 1/4? uhh I’m sorry I don’t know what the remainder is😔
i’m pretending to get it kinda but long division left my head ages ago
😭😓😅☠️whoops
it comes back around to you so I would suggest refreshing on it
ok Imma try to manually do this real quick
omg what
okay no rush you can take your time
lol oh yes
integrals are basically a geomtetric proof, but it's not too bad
if you don't go past calculus then you probably won't use it too much
but in advanced math it rears its head
ok there we go
so you see how our remainder is 1?
yes
so that's how that's related
it's not wrong
but 0.25 is not teh remainder
1 is
bingo
But how does that relate to trigonometry and cosine?
the reason we divide it by 4 is because 4's remainders can only be 0, 1, 2, 3
it just happens that the derivative correlates to the remainder
get a remainder of 1? It's the first derivative
remainder of 2? 2nd derivative
remainder of 3? 3rd derivative
ohhhhhhhh
so it’s sin x?
our original funtion was sin(x)
You said it’s the 1st derivative
ohh
IHHH
OH
Lightbulbs are flickering in my mind right now
okiiii
yes
so what's our derivative?
-cos x
I had to school all the way up to see your picture for that, but I still got the answer right .., :) ?
yep exactly!
see?
super cool right
BUT
this only works if it's the original parent function sin(x) or cos(x) or if it has a scalar like 2sin(x)
but if you were to have something like sin(2x), then you can't use this trick
yes but that means I have to find out the remainder through long division every time, right
So nothing inside the X
yeah exactly
even something like sin(x) + 1 won't work
becuase you lose that + 1 in the derivative
so it's kind of a special case thing
But the pattern is easy than it being something different every time and you having to actually calculate it
yep exactly
really what I wanted to show was that there are sometimes patterns that make your life so much easier
it also shows up in some parts of higher math but it's complex and I don't feel like getting into it right now haha
loll don’t worry u don’t need too but i do appreciate you teaching me this little trick cuz if my teacher ever mentions it imma feel ahead and lowkey like a smarty pants LOL and it’s fun so thank u !
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I need help regarding bounds in a u and v substitute for a double integral im in calc 3 and dont get this part help !!
i will include screenshot of given solution work and circle the part im confused about
does anyone get how he gets the bounds for v???
plzzzz
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@rose sandal Has your question been resolved?
@rose sandal Has your question been resolved?
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Calculator help - idk how to exit this
I just want to exit so i can do general calculations
2nd+quit?
@soft escarp
I mean 2nd+mode (which will quit)
so press 2nd, then press 'mode'
Umm that didn’t really work for some reason
But then i removed the battery
And then put it back
And it reset like a charm
Lol
Well anyhow the problem has been solved!
Thank you for your help : )
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Could someone check my work on this proof?
This is Corollary 2.61
@tiny yew Has your question been resolved?
<@&286206848099549185>
if no one answers this, may have better luck in #numerical-analysis
i would try to answer but i am very eepy
yo @noble laurel someone from your class?
Yes
answer looks good
Yeah I saw that but I kind of did it out of order and went overkill with the order 5 and didn't bother to go back and change it
yeah a bit out of order
usually better to first check how much you have to do before doing it
It wasn't really any extra work because all the summation was on a calculator
I almost did like order 10 before revising down lol
yeah of course. just a general note
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Hey 😃Is this red market step allowed to do ?
p(x) = -2x^2 - 2x + 4
@deft fulcrum Has your question been resolved?
<@&286206848099549185> maybe
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im stuck on the taylors remainder theorem part
ive found (f^3(c))/3! (x^3) but i dont know what to do after
how do you get that ?
what's taylor remainder theorem for you ? do you have a statement of it at hand ?
does that allow you to simplify anything in that expression ?
@olive edge Has your question been resolved?
I mostly think about the x and c
you want the answer to be a plain old number
so you have to manage to get rid of those
by using some inequalities you know (or find) on x and f^(3)(c)
yeah right so we know $|R_n(x)| = \left\lvert \frac{f^{(3)}(c)}{3!} x^3 \right\rvert \leq \frac{1}{48} |f^{(3)}(c)|$
aPlatypus
that 1/48 is just (1/2)^3 * 1/3!
now the problem is that we have to bound f^(3)(c) for c between 0 and 1/2
it's time to get a few more derivatives of f
how did you get to this??
x is between 0 and 1/2
so x^3 is between 0 and (1/2)^3
and the 3! was already there
oh okay
you'll prolly have to get 4 derivatives
what do i do after that?
well try to find what the minima and maxima of f^(3) are
what have you done then ?
found f^(4) and found x when f^(4) equals 0
would the max just be when x=1/2 for f^(3)
if f^(4) is always positive then yea
i have no idea what im doing
well what have you got then ?
like post your working if you're unsure
@olive edge
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i wanted to find the intervals for which f(x) = 4sin^2x + 8sinx + 3 is concave up or down in the intervals 0 <= x <= 2pi, and i found x = pi/6, 5pi/6, and 3pi/2. when i tested it, i found that 5pi/6 < x < 3pi/2 is concave up, and 3pi/2 < x <= 2pi is concave up.
does this mean 3pi/2 is not an inflection point?
0 <= x < pi/6 is concave up
pi/6 < x < 5pi/6 is concave down
so pi/6 and 5pi/6 are the inflection points
but is 3pi/2 an inflection point? since the concavity doesnt change
According to Wikipedia this would be an undulation point instead
@thorn terrace Has your question been resolved?
It's because the lowest-order non-zero derivative above the second has even order
In other words, at x=3pi/2, the second derivative of f is 0, the third derivative is also 0, and the fourth derivative is not
The fourth derivative is of lowest even order after the second
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Question: Take a shuffled deck of 52 cards and throw the first top 10 cards in the bin. what is the probability that the new top card is red?
Is it just 0.5?
As all the scenarios of takiing more red than black should balance out with the scenarios of taking more black than red
that's a good way of thinkiing about it I didn't consider lol
Thanks!
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Can I repoen this one :D
claim a new available channel and repost the question
Arl, thanks!
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@sleek fjord Has your question been resolved?
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not sure where to go from this, can someone help?
how come you found the magnitude of the normal vectors?
tbh i dont even know

write the normal vectors
(2, -1, 4) (-1, 5, -1)?
if the angle between them is zero or pi
i mean, sure but theres a more direct and simple approach
just check if they are multiples of one another
whats the point of confusion?
do you mean if u x v = v x u
is that what you mean if they are multiples of one another
no thats just multiplying vectors
oh no
what i mean is, if you have some vector v
if another vector w = av where a is a scalar, then w and v are parallel
check if they are multiples (scaled versions) of eachother
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hello i need help about differantial equations
Do you have a specific question
i separeted x s and y s than i integrated x's polynoms but i cannot did -y. y' d/dy
Can you show your work
@errant flicker Has your question been resolved?
help me
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Help
What about bottom one
Idk it
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How do I find the roots
Could you think of another way to rearrange the last but one step? where you have -108cos^3+256sin^3=0
no
if you move the cos^3 on the other side, then?
so you have
-108 cos^3 theta + 256 sin^3 theta = 0
am i reading that right?
on 2nd to last line?
yes
instead of last step:
256 sin^3 = 108 cos^3
sin^3 / cos^3 = 108 / 256
tan^3 = 108 / 256
then you can take cube root of both sides and take inverse tangent of both sides to solve for theta
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I'm completely at loss with how to do these questions (I've missed the lesson and the internet isn't really helping),
that's my attempt so far, if you could even call it that.
can someone please explain what it means?
@serene dagger Has your question been resolved?
<@&286206848099549185> sorry to tag you
What are mean reactions at c and d ?
i assume seeing as this model only has "downward" motion (6kg pulling downwards), i only need to look at A and B that's going down respectively?
I will try
I think this is very easy, but I can't help you enough because my English is kind of weak. Use YouTube, that's a good option too
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<@&286206848099549185> sorry to tag
ken la o kepeken ni, ni li lukin pona
https://skyciv.com/docs/tutorials/beam-tutorials/determine-the-reactions-at-the-supports/
A step-by-step guide on how to determine the reactions at the supports in beams, and how to calculate them using SkyCiv Beam Calculator.
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how did they go from first line to second line
Because vector a cross itself is zero(outer product of two vectors which are parallel to each other is zero)
so this is equal to 0?
so how did 1/2 a × 1/13 b become 1/2 × 1/13 |a × b|
The coefficient can be extracted out the product
Because we add the absolute sign,just care its magnitude
Sorry, more corretly,zero vector
oh yeah you're right
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I just wanted to confirm 10pi was the answer
I can ofc explain why
Since its only asking for the differential, we only need to find f'(x)*h correct?
The formula for area of a circle is A=pi*r^2, and the derivative is A = 2pir
Since we need to find f'(x), x being 10, I plugged that in to our derivative
That gives us 20pi. H is 0.5, bc 10.50-10 is .5. We multiply 20pi by h, or 0.5
That gives us 10pi
Given all that, I just wanted a confirmation that was correct
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i need to write this as a fraction. i tried turning it into a series of 35/1000+35/100000......... then into a geometric series but i got it completely wrong
ill recreate my work in ms paint rq
i converted that to a geometric series after but it didnt work
im probably screwing smth up from the very beginning lol
so that last line isn't right
oof
10^(n+2) starts at 1,000 then goes to 10,000
yeah
i think you want to write it where n starts at 0, so you can use the formula for the summation of a geometric series
actually that might not matter
didnt matter
i got it
thanks dude. i really gotta stop speed running this stuff lmao
i even thought of 2n in my mind but it didnt cross my mind just to like, be smart and not dumb (manually writing the series)
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ok nice
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this is more of an inquiry... but for the first one how did they come up with those two points in the blue there... i know it uses the equation but i cant figure out how, and for the second one i kinda get it but why was x just replaced with 0
for the first one they came up with those two points using the y intercept and the gradient
the first point as you can see is the y intercept, 4
i see that
the second point is just the gradient, -2/3 because they moved 2 down and 3 across from the y intercept
ok so
whenever there is y intercept
x always = 0
thats why they substituted it
so it can be like
(0,5)
(0,4)
so when you substitute x for 0 you can find y intercept
and likewise when you substitute y for 0 you can find x intercept
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i dont understand this equation, how can i proof it with induction ?
which part are you having trouble on
do you know what the inductoin hypothesis should be
probably yeah
and what do i have to do with the <
do you know how a proof by induction works
you want to prove this statement for every n, right
yes
and you want to do this using induction on n
yes
do you know what the inductive hypothesis should be
i dont know the english word for it, do you mean n+1 ?
no
describe to the best of your ability how a proof by induction works
and i'll explain it in the terms you use
i already did this
oh
you want to assume the inequality is true for n right
and then use that to prove that it's true for n+1
yes
that k * k! is smaller than (n+1)!
i think you're missing the sum part there
and also you didn't tell me which one that is
i would start like this
okay nvm the last part is wrong
i dont know how to do the inductive step
(n+1)! is not equal to (n+1)*(n+1)!
as you seem to have indicated in the second line
i put n+1 in for all k's and n's
i dont think the logic of your work was thought through very far
can you write the statement you assume is true (for the induction step) and the statement you need to prove from it
if not, you don't know how induction works
why ?
it's not needed
can you not
i tried to use n = n +1 in the second step like in my previous induction.
i dont know why that is false
well
the statement n = n+1 is universally false for all values of n
so that's probably why
does it have to be k * k! < (n + 1 + 1)! ?
when you get rid of the sum entirely that statement makes no sense
can i pretend like the < is a = ?
sort of
if you put an actual sum there
but uh
i think the problem is you're trying to form-fit this to a different problem
you need to take the time to actually understand what a proof by induction is
rather than just trying to play fill-in-the-blank with a problem copied from someone else doing an induction proof
stop using n and n+1 interchangably
(n+1)! is not equal to (n+2)!
the sum on the left is not less than (n+1)!
you know all of these statements are just universally false
always
all the ones circled in red
pretty much the entire "outline" just consists of mathematically false statements
IF the < would be and =, this would be how i do it
all my inductions worked like that before
do you understand why they work
what do you mean ?
do you know why proof by induction is valid
why did someone first decide this was a good way to do mathematical proofs
how do we know it works
we are trying to prove if this equation is true for 1 number (n) it will be true for all other number ( n+ 1)
that sentence makes zero sense
thats how i learned it
why does it make no sense
okay i dont mean it will be true, i mean we try to prove that its true for all other numbers
n+1 is not "all other numbers"
if n is "1 number" then n+1 is also "1 number"
i would recommend reading something like this
and comparing it to the proofs you've seen that use induction to see if you can understand what's happening
isnt that what i did there ?
no, that's not a proof by induction
it might be one small step of a proof by induction
yes thats the beginning step
but it doesn't really have to do with the overall meaning of a proof by induction
i just wanted to know if my start is correct
ok yes your proof will have something similar in it
i just need to simplify this equation till its the same or am i wrong again ?
no, I'm not sure what you mean by the same
right part needs to be the same as the left part
my teacher told me that if i can show that this statement is true, my induction is done
https://www.youtube.com/watch?v=GdM_iA1Zek4&pp=ygUScHJvb2YgYnkgaW5kdWN0aW9u maybe something like this?
Learning Objectives: Prove a family of claims, indexed by the positive integers, using the idea of induction.
Step 1: Write out the Basis Case
Step 2: Assume true at the kth level. This is the induction assumption.
Step 3: Use the induction assumption to show it is true at the (k+1)th level. This is the induction step.
As a bonus we should ...
not this statement
that statement is always true
In this tutorial I show how to do a proof by mathematical induction.
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:)
he i doing the induction like i said
by showing that the statement is true
what else should induction be
I assure you induction is not about "proving" this obviously true statement is true
otherwise there would only ever be one "proof by induction" and it would be the proof that this statment is always true, regardless of what your a_i values or k is
because it is
the proof is of the proposition P(n+1)
which in the case of the video is the statement that the sum of the first n+1 integers is equal to (n+1)(n+2)/2
this is just a tool sometimes used to accomplish that proof
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