用【分部积分法】 ∫ x^2 cosx dx = ∫ x^2 dsinx = x^2 sinx - ∫ sinx dx^2 = x^2 sinx - 2∫ x sinx dx = x^2 sinx - 2∫ x d(-cosx) = x^2 sinx + 2x cosx - 2∫ cosx dx = x^2 sinx + 2x cosx - 2sinx + C
