Triangle

A triangle or trigon is a polygon consisting of three sides connected at three corners. It is one of the basic shapes in geometry and the simplest of polygons. The corners, also called vertices, are zero-dimensional points while the sides connecting them, also called edges, are one-dimensional line segments.

A triangle has three internal angles, each one bounded by a pair of adjacent sides; the sum of angles of a triangle always equals a straight angle.
The triangle is a plane figure and its interior is a planar region.
Sometimes an arbitrary edge is chosen to be the base, in which case the opposite vertex is called the apex; the shortest segment between the base and apex is the height.

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Source: Wikipedia · Image: Wikimedia Commons · Public domain

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