3. If the scalar function u(x, y, z) has continuous derivatives, and the vector differential operator \nabla=\hat{\mathbf{i}} \frac{\partial}{\partial x}+\hat{\mathbf{j}} \frac{\partial}{\partial y}+\hat{\mathbf{k}} \frac{\partial}{\partial z} then prove the vector identity \nabla \times

\nabla \psi=0