a^2≤x^+y^2≤b^2令x=pcosa,y=psinaa≤p≤b,0≤a≤2π∫∫√(x^2+y^2)dxdy=∫[0,2π]da∫[a,b]p*pdp=a[0,2π]*1/2p^2[a,b]=π(b^2-a^2)
