1. In class we have derived the wave eq. for A in the Lorentz gauge \Delta \mathbf{A}+k^{2} \mathbf{A}=-\mu_{0} \mu \mathbf{j}_{c} \text { with } k=\frac{\omega}{c} \sqrt{\varepsilon \mu} \left.\Delta \varphi+k^{2} \varphi=-\rho_{c} / \varepsilon_{0} \varepsilon\right) Derive another wave eq. for the scalar potential in the same gauge. (