Types of Triangles

Isosceles, Equilateral, Scalene, Obtuse...

Triangles can be classified by various properties relating to their angles and sides. The most common classifications are described on this page.

Classifications of Triangles

Equilateral triangle

picture of equilateral triangle

The Equilateral triangle shown on the left has three equal sides and three equal angles.

Each angle is 60°

Isosceles triangle

The Isosceles triangle shown on the left has two equal sides and two equal angles.

Scalene Triangle

The Scalene Triangle has no congruent sides. In other words, each side must have a different length..

Acute Triangle

The Acute Triangle has three acute angles (an acute angle measures less than 90°)

Obtuse Triangle

Obtuse Triangle

The Obtuse Triangle has an obtuse angle (an obtuse angle has more than 90°). In the picture on the left, the shaded angle is the obtuse angle that distinguishes this triangle

Since the total degrees in any triangle is 180°, an obtuse triangle can only have one angle that measures more than 90°.

Practice Problems

Practice 1

What kind of triangle is pictured on the left?

Practice 2

What kind of triangle is pictured on the left?

Practice 3

What type of triangle is pictured on the left?

Since this triangle has two congruent angles and two congruent sides, it is an Isosceles Triangle

Practice 4

Classify the triangle on the left

Obtuse

Assuming the picture is to scale, the triangle on the right is definitely obtuse (see the blue angle) as well as scalene.

Practice 4

Classify the triangle on the left

Obtuse

Assuming the picture is to scale, the triangle on the right is definitely acute as well as scalene.