sqrt(9-x^2) , x = 3 sin(theta)
sqrt ( 9 - [ 3 sin θ ]^2 ) =
sqrt ( 9 - 9sin^2 θ ) =
sqrt [ 9 ( 1 - sin^2 θ ) ] =
sqrt ( 9 * cos^2 θ ) =
sqrt (9) * sqrt ( cos^2 θ ) =
3 cos θ
