Math thread! Come in and have some pi...

[TrueBallerBrah]

If it's the same priority, why can't I just do all multiplication first then all division? Other than obviously getting the wrong answer.

Edit: nvm I figured it out. Lol I feel like an idiot.

[ctownballer04]

If it's the same priority, why can't I just do all multiplication first then all division? Other than obviously getting the wrong answer.

You can, but you haven't written down the problem clearly. You need parenthesis.

The last 3 you have is in the numerator, not the denominator. For it to be in the denominator you need to write the problem as (16*3)/(2*3).
If you do all the multiplication first, you have 16*3*3/2=144/2=72 which is correct.

[Tharsos]

Could someone help me with a statistics problem?

The following table shows the age of a child in months (x) and the hours of sleep the child needs during nap time each day (y)

Age: 1, 3, 6, 9, 12, 18, 24, 36
Nap time: 8, 5, 3.25, 3, 2.5, 2.25, 2, 1.5

The equation of the least squares regression line is y = -0.1351x + 5.2787

No idea even where to start with this one?

What is it you're trying to figure out? How they got the equation for the line? If you read how to do it in your book and then ask specific questions about what you don't understand it's much easier to help.

Am I missing something? I need to solve these as simulations equations or something?

What's the context/model specification? It almost looks like two-staged least squares regression, but not quite. Either way, 2 equations with 2 unknowns is solvable, but then then they give you solutions that don't solve the system? Seems puzzling...

[JDM-EJ1]

What is it you're trying to figure out? How they got the equation for the line? If you read how to do it in your book and then ask specific questions about what you don't understand it's much easier to help.
.

I figured it out. I wasn't following the formula correctly. Thanks for responding any ways though.

[iDontExistHere]

Last semester while taking trig this problem was presented. It was extra credit but few could do it at the time and everyone that did came up with a different answer. I still dont get it. I want picture explanation(im willing to paypal):

[NeoKantian]

Hi guys I have something that's probably a silly question.

Why is 16*3/2*3=72 and not 8? I always thought with PEMDAS multiplication and division were "equal" in terms of priority. Btw, this is with matlab if that makes a difference.

It's an ambiguous statement so it's best avoided and instead use parentheses where needed. It's kind of like in logic with something like P & Q v R. It could be (P & Q) v R or P & (Q v R).

[Kuzon]

mind=blown at the minds in this thread. mirin lmao

[Ewys]

guys im having trouble calculating derivative of 1/sinx

my steps are -
f(x) = 1
g(x) = sinx
f'(x) =
g'(x) = cosx
f'xgx - fxg'x / (g(x))^2= sinx - cosx / sin^2(x)


answer is -(cosx/sin^2x) tho

[YokedBrah]

guys im having trouble calculating derivative of 1/sinx

my steps are -
f(x) = 1
g(x) = sinx
f'(x) =
g'(x) = cosx
f'xgx - fxg'x = sinx - cosx
-------------- ------------
(g(x))^2 sin^2(x)

answer is -(cosx/sin^2x) tho

f'(x) is equal to 0, not 1, since the derivative of any constant function is zero.

[Ewys]

f'(x) is equal to 0, not 1, since the derivative of any constant function is zero.

ohh wow. thanks bro, i thought it jus dissapears LMAO and isnt a value at all

spent 1 hour looking at it.

[YokedBrah]

ohh wow. thanks bro, i thought it jus dissapears LMAO and isnt a value at all

Haha I was wondering if that was a typo. It's zero, so it multiplies sin x and makes that term disappear

[Ewys]

Haha I was wondering if that was a typo. It's zero, so it multiplies sin x and makes that term disappear

repped, u learn this chit in college or?

[charity4thepoor]

Interedesting inequality

If $f(x) = a_nx^n + a_{n-1}x^{n-1}+\cdots + a_1x + a_0$ is a polynomial with real coefficients then

$$\dfrac{1}{\pi}\int_{-1}^1 f(x) \ dx \le \sum_{i=0}^n |a_i|$$

for non latex brahs with white theme(looks like chit on black):

Spoiler!

If is a polynomial with real coefficients then

You can change the 1/pi to 1/2 and it is still true. Follows since $$\max_{x \in [-1,1]}{f(x)} \leq \sum_{i=0}^n |a_i|$$

[NephilimRising]

Can someone explain to me how a = 2Rcos(theta) from the picture? Presumably law of cosines? I've tried law of cosines but still can't get what they are getting. R is the radius of the circle.

This is a stepping stone in a larger problem of finding the potential on the edge of a uniformly charged circular disk (or radius R).

[NephilimRising]

^ when you inscribe a triangle into a semicircle in this manner, the angle at the apex is always pi/2

It really is amazing how much math you have to remember to do physics problems that you would think are trivial.

Thanks man.

[YokedBrah]

repped, u learn this chit in college or?

Thanks brah. Yeah, I did engineering/math in undergrad and I'm in grad school for engineering/math now

[Ewys]

not sure how to solve it?

basically applied chainrule and got

d/dx(e^(-xy)).y/-x

what do?

[YokedBrah]

not sure how to solve it?

basically applied chainrule and got

d/dx(e^(-xy)).y/-x

what do?

In general: given a function z = f(x,y) and a point (a,b) such that c = f(a,b) and f is differentiable at (a,b), here is how to find the equation of the tangent plane (perhaps more aptly called parametrization of the tangent plane) at the point (a,b,c).

I will denote this parametrization by T(x,y). Let d/dx and d/dy denote partial derivatives with respect to x and y, respectively. Then T(x,y) = c + ( d/dx f(a,b) ) * (x-a) + ( d/dy f(a,b) ) * (y-b).

To solve your problem, just let f(x,y) := exp(-xy), compute all relevant terms, and plug them into what I wrote above.

In order to help understand why what I said is true, you should make an analogy with the one-dimensional version of this problem. I.e., given a function y = g(x) differentiable at the point a with b = g(a), the parametrization of the tangent line to the graph of g at the point (a,b) is given by L(x) = b + g'(a) * (x-a).

[IntegralDx]

not sure how to solve it?

basically applied chainrule and got

d/dx(e^(-xy)).y/-x

what do?

The following might help, and avoids the use of the parameterisations used above.

[Ewys]

thanks bro

i got an exam in 12 hours, i still need to know alot of sh!t

ive been having 4 hours of sleep per day for the past 3 days

i did past paper and only got 15/40 and i need 23/40 to pass

should i only get 2 hours of sleep? i want to try do 3 more past papers before i walk into the exam.

PLEASE NEED ADVICE BROS

[planetbro25]

Anybody understand how to do this problem ? 66
http://i.imgur.com/WWYGK5K.jpg

[CandyMane]

-Professor asks us a Stats Question
-We all think the answer is "the sample mean"
-Spend 30 mins thinking among ourselves
-Professor throws smart pen at the wall, ceiling, and his desk because we cant answer
-Is frustrated

Answer ended up being the Population Mean. Anyone else have a teacher rage at the class? The way he worded the question threw me off personally

[Chesssbrah]

thanks bro

i got an exam in 12 hours, i still need to know alot of sh!t

ive been having 4 hours of sleep per day for the past 3 days

i did past paper and only got 15/40 and i need 23/40 to pass

should i only get 2 hours of sleep? i want to try do 3 more past papers before i walk into the exam.

PLEASE NEED ADVICE BROS

All nighters and math exams don't mix all that well, especially since it seems like you have incurred a significant sleep debt. If you really have to do it, I'd recommend sleeping for a multiple of 1.5 hours, e.g. either 1.5, 3, or 4.5 (more the better obviously), so you can awake at the end of a sleep cycle and not in the middle of REM sleep.

You can use http://sleepyti.me to help determine when to sleep

[iDontExistHere]

Can anyone shed some light:
Demonstrate that the cardinality of the negative integers is the same as the cardinality of the
odd integers.

[GaryOatz]

to prove this you have to define a bijection from the odds to the negatives

[iDontExistHere]

Correct I got it all drawn but I'm not seeing how they relate .

[iDontExistHere]

Thanks, I got help from my professor. I was trying from negative to positive & was lost. I happened to see it after 2 minutes of staring working the opposite way -___-

[Chesssbrah]

-Professor asks us a Stats Question
-We all think the answer is "the sample mean"
-Spend 30 mins thinking among ourselves
-Professor throws smart pen at the wall, ceiling, and his desk because we cant answer
-Is frustrated

Answer ended up being the Population Mean. Anyone else have a teacher rage at the class? The way he worded the question threw me off personally

Prof was probably having a really bad day, but throwing pens across the class is extreme lol. I've never had that happen since most profs don't care if you can answer a question posed in lecture or not, as long as you do well on exams.

[SCAR-H]

Next exam is coming soon and while I understand these topics conceptually, I find I have a terribly difficult time recognising what I can say from a given question--which I guess means I don't truly understand it conceptually. Heh...

Example:
Talked to instructor about this one and he warned me about the way I did this. He said we cannot state that c1 or any other concrete c is zero simply because there is a dependence relation. We know at least ONE of them is zero, but we cannot write it. (still a bit confused)
He said it is fine as long as we define an arbitrary c to be zero and solve for its respective vector. That's what I tried here--which I still don't understand because it seems like the same thing to me.


Another:

[SCAR-H]

^

"c_i is nonzero for some i" would be the right way to go about it. Although, you can technically argue "without loss of generality, assume c_1 is nonzero" however, this is an abstraction that your professor may be uncomfortable with. It is an argument by proof "symmetry".

The same argument you give is easily done as:



for 2(b) you may just be able to state

Thanks mayne! Are the steps I took correct? My first exam was covered in "Can't say this yet...", "This is not true.", etc.