∑5n,)Ao, i三】 =g∑/(7cos日7sma)7dyAe i=l 即 小nfx,)da=J∬f(rcos0,rsin)rdrd6 BEIJING UNIVERSITY OF POSTS AND TELECOMMUNICATIONS PRESS 目录上页下页返回结束目录 上页 下页 返回 结束 i i i i i i i n i 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 O