We do a similar process to get the RMS voltage, but we have to square the voltage first (RMS stands for root-mean-squared). So we have the integral of sin^2 (x) from 0 to pi/4 and from 3pi/4 to pi. We then get the average of that (divide by 2 pi again), and finally we take the square root to get our answer.
Steve
I know that to find RMS voltage from peak voltage we use Vp /sqrt(2). When finding RMS from a function though it looks like we square the function?