The first box is substitution, which is directly brought in by the product function, and then the numerator on the denominator is extracted. Dx terminal needs differentiation: dx = d (1/t) =-dt/t 2. In the second box, although the second term is t, it can be converted into: t=x, so the integral interval of the two integrals is the same, and the integral variables are the same. You can add the integrand function before integration.