## penta-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 ppolygon. 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(3n - 1)/2; the first few are 1,
5, 12, 22, ... ## Related category• NOTABLE NUMBERS | |||||

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