Formula

Triangle Angle Sum Property

A triangle is a polygon with three sides and three interior angles. Each side is conventionally labeled with a lowercase letter matching the uppercase letter of its opposite vertex — in ABC\triangle ABC, side aa faces vertex AA, side bb faces vertex BB, and side cc faces vertex CC. The plural of vertex is vertices, and a triangle is named by listing its three vertices (e.g., ABC\triangle ABC).

The three interior angles of every triangle satisfy:

mA+mB+mC=180°m\angle A + m\angle B + m\angle C = 180°

where mAm\angle A is read as "the measure of angle AA." This property holds for all triangles regardless of shape or size, so whenever two angles are known, the third can be found by subtracting their sum from 180°180°.

Image 0

0

1

Updated 2026-05-02

Contributors are:

Who are from:

Tags

OpenStax

Elementary Algebra @ OpenStax

Ch.3 Math Models - Elementary Algebra @ OpenStax

Algebra

Math

Ch.5 Systems of Linear Equations - Elementary Algebra @ OpenStax

Prealgebra

Intermediate Algebra @ OpenStax

Ch.2 Solving Linear Equations - Intermediate Algebra @ OpenStax

Ch.4 Systems of Linear Equations - Intermediate Algebra @ OpenStax

Related
Learn After