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Old 04-09-2017, 04:21 AM   #9426
masque de Z
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Re: The Official Math/Physics/Whatever Homework questions thread

Quote:
Originally Posted by 1989 View Post
Statistics problem that I cant figure out, should not be too tough for you guys...

X and Y are random variables. They are independent and their distribution is equal.

EV(X)=1
Var(X)=2
EV(Y)=1
Var(Y)=2

Z = X*Y

What is the variance of Z?

What formula do I use?

Thanks in advance.
This here being a homework educational value thread probably one must never miss the chance to expand on these things and we can all learn more that way;

For example try also these links with detailed analysis and generalizations

http://www.coe.utah.edu/~cs5960-02/l...s/Lecture2.pdf

http://stats.stackexchange.com/quest...ndom-variables

Look also in the related product entry in the general link

https://en.wikipedia.org/wiki/Variance



https://www.jstor.org/stable/2281592

and


if independent.

notice



https://en.wikipedia.org/wiki/Covariance

This is worthy too

http://www.stat.yale.edu/~pollard/Co...7/Variance.pdf



Additional value

http://www.math.uah.edu/stat/dist/Joint.html

https://en.wikipedia.org/wiki/Product_distribution
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Old 04-27-2017, 03:08 AM   #9427
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Re: The Official Math/Physics/Whatever Homework questions thread

(forgot password to main account, sorry if this is against the rules)

I have a stats problem, happy with any direction people can point me in. For context I've done intro mathematical stats so I know how to do easier functions of random variable questions (CDF method, transformation method, mgf method) but I'm lost here.

Question:

u and v are i.i.d sampled from the uniform distribution [0,1]

if s^2 = (2u-1)^2 + (2v-1)^2 < 1, then x is sampled from the standard normal distribution. Otherwise, x = (2u-1) * sqrt( -2log(s)/s)

My first thought was to try and use the cdf method and then glue them together in a pdf, I'm not getting very far. The other problem I have is that it seems to me that the conditions imposed on s (if the square is less than 1) imply that the 'otherwise' section would result in a negative log.
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Old 04-27-2017, 05:55 AM   #9428
boganomics
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Re: The Official Math/Physics/Whatever Homework questions thread

Oh should have added: how is x distributed?

Anyway, remembered pw to this account
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Old 05-10-2017, 06:59 PM   #9429
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Re: The Official Math/Physics/Whatever Homework questions thread

Not homework but something I wanted to figure out & I'm a complete drooler so maybe you guys could help me out. I have five people processing returns from several (call it 100) different companies every day. The companies take a different amount of time to process so 15 units of company A might take the same amount of time to process as 40 units of company B. Having records of how much of each company everyone did each day how do I tease out who's outperforming & who is just consistently grabbing the companies that get be processed fast to juice their total numbers?
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Old 05-10-2017, 07:45 PM   #9430
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Re: The Official Math/Physics/Whatever Homework questions thread

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Originally Posted by springs View Post
Not homework but something I wanted to figure out & I'm a complete drooler so maybe you guys could help me out. I have five people processing returns from several (call it 100) different companies every day. The companies take a different amount of time to process so 15 units of company A might take the same amount of time to process as 40 units of company B. Having records of how much of each company everyone did each day how do I tease out who's outperforming & who is just consistently grabbing the companies that get be processed fast to juice their total numbers?
It seems that you should be able to get an average processing time by company from the data. And then you can benchmark against that.
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Old 05-13-2017, 09:11 AM   #9431
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Re: The Official Math/Physics/Whatever Homework questions thread

Factorisation:

I am having trouble breaking down this expression in the way that is wanted. I am asked to factorise an expression using the difference of two squares.

(squared = (s))

3x(s) - 27y(s) = ?

The answer is 3(x-3y)(x+3y)

I have tried a lot of different ways of re-expressing it but I can not get the maths to compute to that.
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Old 05-13-2017, 09:40 AM   #9432
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Re: The Official Math/Physics/Whatever Homework questions thread

Let's pull out a 3 first to get 3x^2-27y^2 = 3(x^2-9y^2)

Now, you hopefully know that (a-b)(a+b) = a^2-b^2. If you set a^2=x^2 and b^2=9y^2, I think that you can find some pairs of square roots to find what you need.
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