"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(3n - 1)/2; the first few are 1, 5, 12, 22, ...
Related category NOTABLE NUMBERS
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