求函数f(x,y)=sinx+cosy+cos(x-y),0≤x,y≤π/2的极值

解： ∂f/∂x=cosx-sin(x-y) ∂f/∂y=-siny+sin(x-y) ∂²f/∂x²=-sinx-cos(x-y) ∂²f/∂y²=-cosy-cos(x-y) ∂²f/∂x∂y=cos(x-y) 先求驻点： ∂f/∂x=∂f