Can someone please explain this integration problem to me?

I was looking at the proof of the area of a circle in my book and I understood up to the part y = +/- ((r^{2} - x^{2})^{0.5}) but then the book said to substitue x with rsin(theta).

I understood everything that was done after this but what I dont understand is why rsin(theta) was used instead of rcos(theta). If theta is the angle the radius r makes with the x axis in the 1^{st }quadrant, shouldn't x = rcos(theta)? Can someone explain?

Thanks

Re: Can someone please explain this integration problem to me?

Actually either or can be substituted. The whole idea is to make use of the Pythagorean Identity to simplify the integrand to something that can be integrated.