Maybe everyone else figured this out years ago, but for some reason it never occurred to me till last week.

A shape with many sides is a polygon. A six-sided shape is a hexagon. A five-sided shape is a pentagon. By the same reasoning, a four-sided shape ought to be a tetragon. (Not quadri-, that's Latin.)

A triangle has three corners, so we call it a triangle. It also has three sides, so it should be a trigon. And indeed it is. The science having to do with measuring such shapes is trigonometry.

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