# penta-

'Penta-' is a Greek suffix meaning 'five.' A **pentagon** is a five-sided polygon; its interior angles add up to 540°
and are each 108° in the case of a regular polygon. A **pentahedron**,
such as the Great Pyramid of Gisa, is a five-sided polyhedron.
A **pentagram**, or **pentacle**, is a five-pointed
star formed by the pairwise extension of the sides of a regular polygon
so that each side meets with the next side but one. **Pentagonal numbers** are figurate numbers (numbers that
can be represented by a regular geometric arrangement of equally spaced
points) of the form *n*(3*n* – 1)/2; the first few are 1,
5, 12, 22, ...