3. (?? 4? 4.2) Show that the matrix ($L_{ij}$) for a surface of revolution $x(t, \theta) = (r(t) \cos \theta, r(t) \sin \theta, z(t))$
(Problem 1.2) is
$\frac{1}{\sqrt{\dot{r}^2 + \dot{z}^2}} \begin{pmatrix} \dot{r}\ddot{z} - \ddot{r}\dot{z} & 0\\ 0 & \dot{r}\dot{z} \end{pmatrix}$
