This is the "multiple integral method of substitution" of the "multiple integral part" in Tongji Book. In the calculation method of multiple integrals in the second section, there is the expression "method of substitution with multiple integrals". Specifically, the double integral needs to be multiplied by a Jacobian determinant when changing yuan, and it is generally not taken in the postgraduate entrance examination, but there is no harm in looking at it. At least you know what to pay attention to when changing yuan with multiple integrals.