用极坐标变换原式=∫(0,2π)dθ∫(1,2)ln(r^2)rdr=2π*(1/2)*∫(1,2)ln(r^2)d(r^2)=π*[r^2*ln(r^2)-r^2]|(1,2)=π*(4ln4-4+1)=(4ln4-3)π
