# Math Help - finding variance of truncus pdf

1. ## finding variance of truncus pdf

I know that E(X^2) is the definite integral of x^2 * f(x).

But what if f(x) = (4/3)(x^-2) ? The answer in the texbook says it is [4x/3].

I'm so confused. Thanks in advance

2. Originally Posted by freswood
I know that E(X^2) is the definite integral of x^2 * f(x).

But what if f(x) = (4/3)(x^-2) ? The answer in the texbook says it is [4x/3].

I'm so confused. Thanks in advance
There is something wrong with the question. Your f(x) is not a density without
a statement about its domain, which in this case would probably be
(4/3, infty). Then as the integration "integrates out" the dependency on x,
what you report as the textbooks answer cannot be the answer to the
question you have posted.

RonL