f(x,y)=x^2/4+y^2/9 ; (-2,-3) 切平面与法线的图像,请问x^2/4+y^2/9=2 的图像是椭圆么?求这个题目计算和图像,

f(x+y)=f(x)+f(y)+2xy f(x+y,x-y)=(x^2-y^2)/2xy,求f(x,y)! 设f(x+y,x-y)=x^2-y^2,则f(x,y)= 若f(x+y,y/x)=x^2-y^2,则F(X,Y)= 已知f(x+y,y/x)=x^2-y^2,则f(x,y)= f(x+y,y/x)=x^2-y^2求f(x,y) f(x+y,y/x)=x^2-y^2,求f(x,y),详见里面 f(x+y,x-y)=2xy(x-y),求f(x,y) f(xy,x-y)=x^2+y^2+xy;f(x,y)/∂x;f(x,y)/∂y f(x+Y)+f(x-y)=2f(x)f(Y) 求其是偶函数 急 求证f（x+y）+f(x-y)=2f(x)f(y)是周期函数 (X,Y) f(x,y)={12y^2,0 邱下列函数的极值:(1)f(x,y)=4-x平方-y平方 (2)f(x,y)=x平方-xy+y平方+9x-6y+20 (3)(3)f(x,y)=x的三次方-3xy+3y(4)f(x,y)=4(x-y)-x平方-y平方 f(x^2-y^2,x-y)=x+y+x^2-xy,求f'(x,y)对y求导 已知F(X+Y,X-Y)=X^2*Y+X*Y^2,则F(X,Y)=? 设f(x+y,x-y)=x^2+y^2-x*y求f(x,y) 设f(x)=e的y次方,证明：（1）,f(x)f(y)=f(x+y) ,(2),f (x)/f(y)=f(x-y) f[(x+y)/2]