求导数f(x,y)

∂f/∂x=[kcosxcosysin(x-y)-ksinxcosycos(x-y)]/sin²(x-y)=-½ksin2y/sin²(x-y)∂f/∂y=[-ksinxsinysin(x-y)+ksinxcosycos(x-y)]/sin²(x-y)=½ksin2x/sin²(x-y)全微分：df=[½k/sin²(x-y)](-sin2ydx+sin2xdy)