  # Finding the Third Angle of a Triangle

In this worksheet, students find the third angle of a triangle given the other two angles. Key stage:  KS 3

Curriculum topic:   Geometry and Measures

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

Difficulty level:   #### Worksheet Overview

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°

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