Right Triangles 

A right triangle is a triangle that has a right angle (i.e., a 90° angle). 

Terminology 

The side opposite of the right angle is called the hypotenuse. The other two sides are called legs. 

An Important Property of Right Triangles

In a right triangle, the sum of the squares of the lengths of the two legs is the square of the length of the hypotenuse. 

This property is the famous Pythagorean Theorem (or Pythagoras’ Theorem).  If the lengths of legs are denoted by a and b and the length of the hypotenuse by c, we have a² + b² = c². 

Other Properties

1. A right triangle must have exactly one right angle.
2. The other two angles must be acute and are complementary (i.e., add up to 90°).