Concept

● An angle is a figure formed by two rays, called the sides of the angle, sharing a common endpoint, called the vertex of the angle.
● The sum of the angles in a triangle is always 180°.
● Supplementary angles are two angles whose sum is 180 degrees
● Complementary angles are two angles whose sum is 90 degrees.
● Vertical angles are the angles opposite each other when two lines cross. They are always equal.

Rule

1. The sum of the three angles must be 180°. 
2. Write an equation.
3. Solve for the measure of the unknown angle

Example

Find the measure of the missing angle. 

Solution

1. The sum of the three angles must be 180°. The two angles given measure 39° and 79°.

2. Write an equation and find the measure of the unknown angle.
     
39°  +  79°  +  ? =  180°

118°  +  ?  =  180°
  
?  =  180°  − 118°

?  =  62°

Answer: 62°

Pre-requisite Skills

Measuring Angles (4.10.1)