giải hộ em câu n với ạ

Ta có :

`W''_x = \frac{\partial W}{\partial x} = \frac{\partial}{\partial x}(\sin^2 x + 6x \cos y)= 2\sin x \cos x + 6\cos y`

`W''_y = \frac{\partial W}{\partial y} = \frac{\partial}{\partial y}(\sin^2 x + 6x \cos y)= 0 + (-6x \sin y) = -6x \sin y`

 `->`$W''_{xx}$ `= \frac{\partial}{\partial x} ( \frac{\partial W}{\partial x} )=\frac{\partial}{\partial x}(2\sin x \cos x + 6\cos y)=2\cos 2x`

`->W''_{yy} = \frac{\partial}{\partial y}( \frac{\partial W}{\partial y})= \frac{\partial}{\partial y}(-6x \sin y)= -6x \cos y`

`->W''_{xy} = \frac{\partial}{\partial y}( \frac{\partial W}{\partial x} )=\frac{\partial}{\partial y}(2\sin x \cos x + 6\cos y)=-6\sin y`

`->W''_{yx} = \frac{\partial}{\partial x} ( \frac{\partialW}{\partial y} )= \frac{\partial}{\partial x}(-6x \sin y)= -6 \sin y`