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A '''triangle''' is a three-sided figure.
A '''triangle''' is a three-sided figure, made by any three points that are not colinear.
In [[Euclidean geometry]], each side of a triangle is perfectly straight, and 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]]).
An [[isosceles triangle]] has two equal angles, and two equal sides.
An [[equilateral triangle]] has three equal sides, and three 60º angles.
If a triangle is not one of the above, it is a [[scalene triangle]].
==Congruence of triangles==
==Congruence of triangles==