A triangle is a polygon with three sides and is determined by three line segments called sides, or three nonaligned points called vertices.

The vertices of a triangle are written with capital letters.

The sides of a triangle are lowercase and correspond with the letters of the opposite vertices.

The angles of a triangle can be written like the vertices or with Greek letters.

Properties of Triangles


One side of a triangle is less than the sum of the other two and greater than their difference.

a < b + c

a > b − c


In a triangle, the greatest angle is always opposite the longest side.


 If a triangle has two equal sides, its opposite angles are also equal.

Equal Triangles


Two triangles are equal when they have the same sides and two adjacent angles match.

2Two triangles are equal when they have two equal sides and the angle between them is the same.


Two triangles are equal when the three sides are equal.

Types of Triangles by Sides

Equilateral Triangle

Three equal sides.

Isosceles Triangle

Two equal sides.

Scalene Triangle

Three unequal sides.

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