# QM - variance

1. Sep 23, 2007

### cscott

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

Compute the variance of the random variable X given by

$$V(X) = \sqrt{E((X-E(X))^2)}$$
where E(X) is the expectation value of random variable X

2. Relevant equations

Hint: Use parameter differentiation

3. The attempt at a solution

I have no idea what to do here. I've never taken a class in probability and I have never heard of parameter differentiation. I've seen definitions of variance the same as above minus the square root sign so I'm confused.

Last edited: Sep 23, 2007
2. Sep 23, 2007

### malawi_glenn

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

look at "Computational formula for variance"

hmm you should have no square root; the formula you have is the standard devation S(x)

S(x) = (V(x))^(1/2)

At least what I know of statistics.

Use that E is linear operator (E : expectation value)

But I must say that it is hard so see what is meant by the problem..

Last edited: Sep 23, 2007