>>95
じゃあ定義に従ってxとyの両方で偏微分すればいいと思うが
∂f/∂x=cos(x+y)cos(x-y)-sin(x+y)sin(x-y)=cos2x
∂f/∂y=cos(x+y)cox(x-y)+sin(x+y)sin(x-y)=cos2y