In this page we practice using the rule:
Sum of interior angles of a triangle = 180°
Example 1
Angles of a triangle are:
40°, 60°, and x.
40 + 60 + x = 180
100 + x = 180
x = 80°
Example 2
Angles of a triangle are:
90°, 35°, and x.
90 + 35 + x = 180
125 + x = 180
x = 55°
Example 3
Two angles of a triangle are equal and one angle is 40°.
Let the equal angles be x.
x + x + 40 = 180
2x + 40 = 180
2x = 140
x = 70°
So the triangle has angles:
70°, 70°, and 40°.
Example 4
Angles of a triangle are:
x, x, and x.
x + x + x = 180
3x = 180
x = 60°
So all angles are 60°.
Example 5
Angles of a triangle are:
80°, 70°, and x.
80 + 70 + x = 180
150 + x = 180
x = 30°
Practice
- More Examples
- Take the Quiz
Continue Learning
- Types of Triangles
- Triangles (Basics)
- Sum of Angles of a Triangle is 180° (Proof)
- Find the Missing Angle of a Triangle
- Sum of Exterior Angles of a Triangle (Proof)
- Exterior Angle Theorem of a Triangle
- Two Exterior Angles at a Vertex are Equal (Proof)
- Two Exterior Angles are Equal at a Vertex (Quick Proof)