A five-sided polygon is called a pentagon. A pentagon also has five vertices and five interior angles, which add up to a total of 540 degrees.

The perimeter of a pentagon is calculated by adding the length of all the sides together. A regular pentagon has all sides of equal length and all interior angles measuring 108 degrees. A regular pentagon is able to be divided into five regular triangles. Finding the area of one of the triangles and multiplying it by five yields the area of the regular pentagon. A real-life example of this five-sided figure is the Pentagon in Arlington, Va., located right outside of Washington, D.C. This five-sided building houses the U.S. Department of Defense.