练习14 2.∵山=P(x,y)dx+Q(x,y)dy, a f P(x,y)=a, o(x, y) af 故 aP 82f 8g a2f ay axay ax ayax OP 00 ay ax K心练习1.4 一. 2. df = P(x, y)dx + Q(x, y)dy, ( , ) , xf P x y   = ( , ) , yf Q x y  = , 2x yf yP    =  故 , 2y xf xQ    =  . xQ yP  =  