In this page we practice finding missing angles using the rule:

Sum of interior angles = 180°

Example 1

A = 60°
B = x
C = 30°

60 + x + 30 = 180

x = 90°

Example 2

A = x
B = 90°
C = 40°

x + 90 + 40 = 180

x + 130 = 180

x = 50°

Example 3

A = 50°
B = 100°
C = x

50 + 100 + x = 180

150 + x = 180

x = 30°

Example 4

A = x
B = 60°
C = x

x + 60 + x = 180

2x + 60 = 180

2x = 120

x = 60°

Example 5

D = x
E = 90°
F = x − 20°

x + 90 + (x − 20) = 180

2x + 70 = 180

2x = 110

x = 55°

Example 6

G = x
H = 3x
I = 2x

x + 3x + 2x = 180

6x = 180

x = 30°

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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