1993 AIME Problems/Problem 10

Problem

Euler's formula states that for a convex polyhedron with vertices, edges, and faces, . A particular convex polyhedron has 32 faces, each of which is either a triangle or a pentagon. At each of its vertices, triangular faces and pentagonal faces meet. What is the value of ?

Solution

The convex polyhedron of the problem can be easily visualized; it corresponds to a dodecahedron (a regular solid with equilateral pentagons) in which the vertices have all been truncated to form equilateral triangles with common vertices. The resulting solid has then smaller equilateral pentagons and equilateral triangles yielding a total of faces. In each vertex, triangles and pentagons are concurrent. Now, the number of edges can be obtained if we count the number of sides that each triangle and pentagon contributes: , (the factor in the denominator is because we are counting twice each edge, since two adjacent faces share one edge). Thus, . Finally, using Euler's formula we have .

In summary, the solution to the problem is .