In the parametrization (psi=A exp left[frac{i}{hbar} W(vec{r})-frac{i}{hbar} E t ight]), what is the equation of motion for

In the parametrization \(\psi=A \exp \left[\frac{i}{\hbar} W(\vec{r})-\frac{i}{\hbar} E t\right]\), what is the equation of motion for \(A\) ? What happens to it in the geometrical optics approximation? If also \(V(\vec{r})=0\), solve the equation for \(A\).