cos2x=cos*2x-sin*2x cot*2x=csc*2x-1 cotx的导数=-csc*2x
∫（cos2x／sin^2x）dx=∫cos*2x-sin*2x／sin^2xdx 经过化简得 =∫cot*2xdx-∫1dx =∫csc*2xdx-∫1dx-∫1dx =-cotx-2x+c�