An eighteen-sided polygon is called an octadecagon or octakaidecagon, from the Greek words okta, meaning 8, the modifier -kai-, the word deka, meaning 10, and the suffix -gon, meaning angle.
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The name of a polygon with 15 sides is a pentadecagon
18
A pentagon is the polygon with 5 sides.
A polygon with seven sides is a heptagon.
360/18 = 20 sides