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Reference

Triangles

Concept

Triangle

A triangle is a polygon with three angles and three sides. The symbol is used to algebraically denote a triangle. Next to this symbol, the three vertices of the triangle are written in any order. For example, and both denote the triangle whose vertices are and
Interactive triangle with movable vertices
Triangles can be classified either according to their side lengths or to their internal angle measures.
Interactive triangle with a movable vertex
The following table lists all the different types of triangles according to their corresponding classification.
Classification of Triangles
By Angle Measures By Side Lengths
Acute Triangle Scalene Triangle
Right Triangle Isosceles Triangle
Obtuse Triangle Equilateral Triangle
Equiangular Triangle

Concept

Scalene Triangle

A scalene triangle is a triangle in which all three sides have different lengths. In other words, it has no congruent sides.
Interactive triangle with a movable vertex
Additionally, in a scalene triangle, the three angles have different measures. In other words, scalene triangles have no congruent angles.

Concept

Isosceles Triangle

An isosceles triangle is a triangle that has exactly two sides of the same length. In other words, it has two congruent sides. The congruent sides are called legs while the third side is called the base. The angle between the legs is called the vertex angle and the other two angles are called base angles.
Interactive triangle with a movable vertex
In an isosceles triangle, the base angles are congruent angles — they have the same measure.

Concept

Equilateral Triangle

An equilateral triangle is a triangle in which the three sides have the same length. In other words, the three sides are congruent.
Interactive triangle with a movable vertex
In an equilateral triangle, the three angles have the same measure. Therefore, equilateral triangles are also equiangular triangles.

Concept

Acute Triangle

An acute triangle is a triangle where all angles are acute. In other words, the three angle measures are less than
Acute triangle vs. Non acute triangle
In case the angles are measured in radians, a triangle is acute when each of the angle measures is less than or about radians.

Concept

Obtuse Triangle

An obtuse triangle is a triangle that has one obtuse angle. In other words, one of the angles measures more than
obtuse triangle vs. Not an obtuse triangle
In case the angles are measured in radians, a triangle is obtuse when one of the angles measures more than or about radians.

Concept

Right Triangle

A right triangle is a triangle that has one right angle. The side opposite the right angle is always the longest and is known as the hypotenuse. The other sides are commonly called legs. Notice that in a right triangle, the legs are perpendicular to each other.
Right triangle vs. Not a right triangle
If one of the acute angles in the triangle is labeled, the legs can be discussed relative to that angle. Consider an acute angle labeled as on the diagram. The side that forms the angle is the adjacent side and the side not touching the angle is the opposite side.
Sides of a right triangle labeled
Note that the opposite and adjacent sides change when changes, but the hypotenuse is always the same.

Concept

Equiangular Triangle

An equiangular triangle is a triangle in which the three angles have the same measure. In other words, the three angles are congruent. In fact, the three angles measure each. If the angles are measured in radians, then the three measures are equal to
Interactive triangle with a movable vertex
Notice that an equiangular triangle is also an acute triangle. In an equiangular triangle, the three sides have the same measure. Therefore, equiangular triangles are also equilateral triangles.