P$2\mathrm{.}26$ A uniform bar with length c can slide inside a cylindrical surface with a radius of $r.$ Determine the maximum angle $$ that guarantees the equilibrium of the bar if the friction coefficient at the points of contact is $.$

$2\mathrm{.}26=ta{n}^{-1}(\frac{r^2}{r^2-\frac{c^2}{4}(1+{}^2)})$

Figure P$2\mathrm{.}26$