设 fi 为f对第i个变量的偏导,i = 1,2,3
dz -f1(2x,x+y,yz)*2dx -f2(2x,x+y,yz)(dx+dy)-f3(2x,x+y,yz)*(ydz+zdy) = 0
==>
dz = ((2f1(2x,x+y,yz) - f2(2x,x+y,yz))dx +(f2(2x,x+y,yz) +zf3(2x,x+y,yz))dy)/(1-yf3(2x,x+y,yz))