Draw a straight line parallel to the number axis. If dx integrates first, draw a straight line parallel to X. The two intersections of the line and the integrated area will integrate the upper and lower limits, and then dy will integrate, and the range of values is the maximum and minimum values of the area. If you integrate dy first and draw a straight line parallel to the y axis, the process is the same.

Change the order of integrals and exchange which process is taken first.

Use polar coordinates to directly set the formula, what X becomes, what Y becomes, and then the polar axis value range and angle range.