In this page we practice using the idea that:

Each exterior angle = 180° − interior angle

So both exterior angles are equal.

Example 1

Interior angle = 60°

Exterior angle = 180 − 60

Exterior angle = 120°

Both exterior angles = 120°

Example 2

Interior angle = 80°

Exterior angle = 180 − 80

Exterior angle = 100°

Example 3

Interior angle = x
Exterior angle = 2x − 20

180 − x = 2x − 20

180 + 20 = 3x

200 = 3x

x = 66.67° (approximately)

Example 4

Interior angle = x
Exterior angle = x + 40

180 − x = x + 40

180 − 40 = 2x

140 = 2x

x = 70°

Practice

• More Examples

• Take the Quiz

Continue Learning

1. Types of Triangles
2. Triangles (Basics)
3. Sum of Angles of a Triangle is 180° (Proof)
4. Find the Missing Angle of a Triangle
5. Sum of Exterior Angles of a Triangle (Proof)
6. Exterior Angle Theorem of a Triangle
7. Two Exterior Angles at a Vertex are Equal (Proof)
8. Two Exterior Angles are Equal at a Vertex (Quick Proof)

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