You need to be clear what it is that you want.
For some applications the average value of a function f over an interval is the integral of f over the interval divided by the length of the interval.
The "mean" of a distribution function (for which the integral over the interval is 1), is the integral of xf(x) over the interval.
Just a little further help the integral of x1/2 is (2/3)x3/2.