lim ∑f5k,7)△ok 2→ k=1 =lm2f(Aeos8,Asna)p4p,A0 即 f(x)do-f(pcos0,psin0pdpdo pd0 0 1 lim ( cos , sin ) n k k k k k k k k f         → = =    D 即 f (x, y)d ( cos , sin )   d d D = f      d  d  d d