Let’s start our study of triangles with discussing the three main types of angles. These types are *acute angles,* *right angles,* and *obtuse angles*.

DEFINITION |

An **acute angle** is an angle with a measure less than 90º.

A **right angle** is an angle with a measure of 90º.

An **obtuse angle** is an angle with a measure greater than 90º.

Recall from geometry that you need three known measures to describe a triangle—the lengths of two sides and the measure of an angle, the lengths of three sides, or three other measures. For a right triangle, you need only two additional measures, because you already know that one of the angles’ measures is ...

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