# Problem with Statistics/probability/calculus?

1. Nov 7, 2007

1. The problem statement, all variables and given/known data

let X denote the vibratory stress (psi) on a wind turbine at a certain wind speed. the pdf is

f(x;y) = (x/y^2)*(e^((-x^2)/(2y^2))

a) i need to prove that it is a pdf.
b)assume y=100, what is the probabilty X is at most/least 200? whats prob X les than 200

2. Relevant equations

3. The attempt at a solution

so i did part a part 1...f(x;y) is greter than 0 for all values...
but for the integral of f(x)=1, i think i need to know how to integrate that, and i have no idea how!

2. Nov 7, 2007

### Dick

Try the substitution u=x^2/(2*y^2).

3. Nov 7, 2007

y is a constant tho?

4. Nov 7, 2007

### EnumaElish

I think y is constant.

5. Nov 7, 2007

### Dick

Sure. You said you were integrating dx, right? y is just a parameter.