Hi there any help showing the below integral equals the function +C?

integral= sqrt[(a^2-x^2]dx

function=(a^2/2)(theta+sin(theta)cos(theta))+c

using Substitution x=(a)sin(theta).

tried using

integral dx/sqrt[(a^2-x^2]=theta + C but can't seem to crunch it!

used dx=acos(theta).

any help appreciated.