How to integrate sqrt(a^2-x^2) from 0 to a

im not sure how to do this ? im pretty sure is trig substitution ?
steps would be appriciated !!
thanks in advance :D

sqrt(a^2-x^2) = (a)sqrt(1-(x/a)^2)

Let x/a = sin(u)

dx = (a)cos(u)du

1 - sin^2(u) = cos^2(u)

so the integral becomes (a )cos^2(u) du from u = 0 to pi/2

You can find the solution for this integral at http://calc101.com/special_2.html