i∑f(5k,mk)Ak →>0 k=1 lim ∑f( k cos ek, rk sin ek)△Vk△Ok ->0 即 f(r, y)do=l f(rcos e, rsinO)rdrde D rde O d Q团pk k k k k k k n k f r  r  r r   =    = → lim ( cos , sin ) 1 0 D 即 f (x, y)d r d r d  = D f (r cos,rsin ) d r d r rd d