GRANULAR ANGLE OF REPOSE Different types of granular substances naturally settle at different angles when stored in cone-shaped piles. This angle $\theta$ is called the $angle\ of\ repose$ (see figure). When rock salt is stored in a cone-shaped pile 11 feet high, the diameter of the pile's base is about 34 feet. (Source: Bulk-Store Structures, Inc.)
(a) Find the angle of repose for rock salt.
(b) How tall is a pile of rock salt that has a base diameter of 40 feet?