Hello, just a question.

Find the cartesian equation of the curves whose parametric equations are:

a) x = 2cos(theta), y = 2sin(theta), 0 ≤ theta ≤ pi

the answer is: y = sqrt( 4 - x^2), -2 ≤ x ≤ 2

I understand how to do it up to x^2 + y^2 = 4, and I can arrive at the answer, but I do not knowWHYthe solution is y = sqrt( 4 - x^2), -2 ≤ x ≤ 2. Something to do with the domain, but I dont understand.

Thanks.