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Finding the Third Angle of a Triangle

In this worksheet, students find the third angle of a triangle given the other two angles.

'Finding the Third Angle of a Triangle' worksheet

Key stage:  KS 3

Curriculum topic:   Geometry and Measures

Curriculum subtopic:   Derive/Use the Sum of Angles for Deduction

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

The three interior angles of any triangle add up to 180°. If we know two, we can find the third.

 

Examples

a + 29° + 122° = 180°
a = 180°  - 151 = 29°

b + 48° + 90° = 180°
or b + 48° = 90°
b = 42°


c = 60° (isosceles triangle)
d + 2×60° = 180°
d = 60°

Triangle is equilateral


f + 94° = 180° (angles on the straight line)
f = 86°
86° + 35° + e = 180°
e = 59°

---- OR ----

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