Lesson 1: Classify Triangles

Property

Triangles can be classified by the lengths of their sides:

• All three sides of an equilateral triangle are equal in length.
• In an isosceles triangle, two sides are equal in length.
• In a scalene triangle, all three sides have different lengths.

Examples

• A triangle with side lengths of 7 cm, 7 cm, and 7 cm is an equilateral triangle.
• A triangle with side lengths of 5 inches, 5 inches, and 8 inches is an isosceles triangle.
• A triangle with side lengths of 4 m, 6 m, and 9 m is a scalene triangle.

Explanation

A triangle's name can also come from its side lengths. 'Equilateral' means all sides are equal. 'Isosceles' means two sides are equal. 'Scalene' means no sides are equal—they all have different lengths.

Property

Triangles can be classified by the specific sizes of their interior angles into three categories:

• Right Triangle: Contains exactly one right angle (90°).

• Obtuse Triangle: Contains exactly one obtuse angle (greater than 90°).