∂f/∂x=fx=-4cos(2x+y)+sin(x-y)
∂f/∂y=fy=-2cos(2x+y)-sin(x-y)
∂²f/∂x²=fxx=8sin(2x+y)+cos(x-y)
∂²f/∂y²=fyy=2sin(2x+y)+cos(x-y)
∂²f/∂x∂y=fxy(原题这里写错！)=4sin(2x+y)-cos(x-y)
∂²f/∂y∂x=fyx(原题这里写错！)=4sin(2x+y)-cos(x-y)
如果这里的变量是xy两个，那么2阶导数有三个：分别是：dx2 dy2 dxdy
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