This paper studies the substitution integral method. Dt stands for derivative of t, and t is cosx. So dt is taking the derivative of cosx, which is -sinx. Because the derivative of cosx is -sinx. The derivative of sinx is cosx, so pay attention to the sign of the derivative here. The following steps use the formula derived from the partial integral of definite integral, and the integral result can be calculated quickly according to the formula. In some places, online classes are spoken too fast, and we need to read books to understand them deeply. I sometimes watch online classes, but if I watch online classes, the formula is incomplete and the knowledge points are not comprehensive. I still have to read books to make up for the lack of online courses.