Let the random variable X have the probability distribution function

f(x)={█(2/(√2π) e^(〖-x〗^2/2)@0             otherwise)┤for 0<x<∞
Find the mean and variance of X
The hint says, compute E(X) directly and then compute E(X2) by comparing that integral with the integral representing the variance of a variable that is N (0, 1)