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A triangle is a three-sided figure. In geometry, each side of a triangle is perfectly straight.  

A triangle is a three-sided figure.  

The sum of the internal angles of a triangle is always 180º.

In Euclidean geometry, each side of a triangle is perfectly straight, and the sum of the internal angles of a triangle is always 180º.

A [[right triangle]] has one 90º angle. Right triangles have special properties (see [[trigonometry]]).  

A [[right triangle]] has one 90º angle. Right triangles have special properties (see [[trigonometry]]).