利用极坐标系计算二重积分 △a1=Gr+△)·△6 2 r:·△ 6=6.+△6 r+△ (2r1+△r)△r·△6 △o r+(r2+△ △r:△ 2 F·Ar·△日, A fs(x, y)dxdy=f(rcos B, rsin O)rdrd@A o D  i i r = r i i r = r + r  =  i +  i  =  i i i i i i i  = r + r   − r   2 2 2 1 ( ) 2 1 i i i i = (2r + r )r   2 1 i i i i i r r r r    + +  = 2 ( ) , i i i = r  r   ( , ) ( cos , sin ) .   = D D f x y dxdy f r  r  rdrd 三、利用极坐标系计算二重积分