Moment generating function?

[jer4011]

I need help solving what appears to be a fairly simple problem.
Find the moment generating function of :
fx(x)=xe^-x

Can anyone help me get going in the right direction. I also need to find the variance after that.
Thanks

[jer4011]

I am pretty sure I just figured it out.

I came up with 1/(1-t)^2. And I know how to get the variance, will do that later.

[curquht]

I am pretty sure I just figured it out.

I came up with 1/(1-t)^2. And I know how to get the variance, will do that later.

I disagree...

I came up with M'(t) = x^2 * e^x(t-1)
then E[x] = M'(0)

var(X) = E[x^2] - (E[x])^2 where E[x^2]= M''(0)

[mp83]

Well, f is Gamma (2,1) so its mgf is (1-t)^-2

[curquht]

wooops~! My bad.

[jer4011]

Thanks for your input, this is the first math class I have ever really had any trouble in.