First go to the polar coordinate system integration → then change the definite integral of θ into the second kind of curve integral (at the first mark) → Green's formula (at the second mark) changes the second kind of curve integral into the double integral → changes the rectangular coordinate system double integral into the polar coordinate system double integral (at the third mark, ρ here is the same as the previous R, just to distinguish the integral variables). Refer to the following figure for specific analysis: