f(x,y)=x^2-2xy+y^2在点(2,1)处的最大方向导数为多少?正在考试,

f(x+y)=f(x)+f(y)+2xy f(xy,x-y)=x^2+y^2+xy;f(x,y)/∂x;f(x,y)/∂y f(x+y,x-y)=(x^2-y^2)/2xy,求f(x,y)! f(x+y,x-y)=2xy(x-y),求f(x,y) f(xy,x-y)=x^2+3xy+y^2,则f(x,y)=? f(xy,x-y)=x^2-xy+y^2.求f(x,y) y'=-2xy;求f(x). 求二元函数f（x,y）=xy/x+y^2在点（1,1）的偏导数 函数f(xy,x+y)=x^2+xy+y^2,则df(x,y)=? 设f(x+y,x-y)=x^2+xy,求f(x,y) 求f(x,y)=xy(x^2+y^2-1)的极值和极值点, f(x^2-y^2,x-y)=x+y+x^2-xy,求f'(x,y)对y求导 二元函数f(x,y)=x^2+xy+y^2-3x-6y的驻点 已知二元函数f(xy,x+y)=x^2+y^2,求f(x,y) 请阐明一下与这个式子在解法上的不同之处f(x+y,xy)=x²+y²，求f(x,y) 设f(x+y,xy)=x^2+y^2-xy,则（∂f(x,y)）/∂y= f(2x+y,2y+x)=xy,求f(x,y) 设f（x+y,xy）=x^2+y^2,则f（x,y） 证明二元函数不可微设f(x,y)=xy/√x^2+y^2,(x,y)≠(0,0)0,(x,y)=(0,0)证明f(x,y)在点(0,0)不可微.