int x * sin (x^2) dx = int sin (x^2) d(1/2 * x^2) = 1/2 * int sin (x^2) d(x^2) =
= | x^2 = t | = 1/2 * int sin t dt = -1/2 * cos t + C = | t = x^2 | =
= -1/2 * cos (x^2) + C