A triangle is a plane shape that has three sides and three angles. Triangles are usually named based on either their sides or their angles.

Isosceles, equilateral, and scalene triangles are examples of triangles named based on their sides, while acute, obtuse, and right triangles are triangles named based on their angles.

opposite sides and angles of a triangle

For a triangle, a side and an angle are opposite each other if the other two sides are sides of the angle. In the figure below, line segment AB is opposite angle C.

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The tricycle in triangle means three. Related words are triathlon, tripod, triplet, and tri-. A triangle is often used in construction, an example being the bridge shown below, because it is a very strong shape.

Related word triangular, triangulate.

See also polygon, acute triangle, equilateral triangle, isosceles triangle, obtuse triangle, right triangle, scalene triangle.