When all obstacles are convex polygons we can improve the shortest

path algorithm by only considering common tangents rather than all

visibility edges.

a$.$ Prove that the only visibility edges that are required in the shortest

path algorithm are the common tangents of the polygons.

b$.$ Give a fast algorithm to find the common tangents of two disjoint

convex polygons.

c$.$ Give an algorithm to compute those common tangents that are also

visibility edges among a set of convex polygons.