Unit A: Geometry

Chapter 1: Polygons

Types of Triangles

Triangles are classified using two main methods: by angles or by side lengths. Every triangle can be classified both according to angles and side lengths.

When classified by angles, triangles can be considered right, acute or obtuse.

Triangles Classified by Angle

Triangle Name	Definition	Diagram	Example of Use
Right	One angle is 90°.
Acute	All three angles are less than 90°.
Obtuse	One angle is greater than 90°.

When classified by side length, triangles can be considered equilateral, isosceles, or scalene.

Triangles Classified by Side Length

Triangle Name	Definition	Diagram	Example of Use
Equilateral	All three sides are equal length. Note: All three angles are equal.
Isosceles	Two sides are equal in length. Note: The angles opposite the two equal sides are also equal.
Scalene	No sides are equal in length. Note: No angles are equal in length.