多元函数f(xy,x-y)=x²+y²,则af(x,y)/ax+af(x,y)/ay=?

af(x,y)/dx+af(x,y)/dy=2x+2y函数f(x-y，x+y)=2x²-2y²。af(x,y)/ax+af(x,y)/ay=？af(x,y)/dx+af(x,y)/dy=4x-4y确定？这个题给的答案是2(x+y)，上个题是2y+2。你给的偏倒求的是一阶的呀！