# Calculate Missing Angles in Triangles and Straight Lines

In this worksheet, students will calculate angles on straight lines and in triangles.

Key stage:  KS 3

Curriculum topic:   Geometry and Measures

Curriculum subtopic:   Apply Facts About Angles and Sides

Difficulty level:

#### Worksheet Overview

In this activity, we are going to look at angles on straight lines and in triangles.

You will be using written methods for adding and subtracting.

In order to do this activity we need to look at three rules.

(You should make a note of the main points to help you with the questions later).

Rule 1

Angles on a straight line add up to 180o

Rule 2

Angles in a triangle add up to 180o

Rule 3

Vertically opposite angles are equal.

How would we use this information?

Well, it means if we are given the sizes of some angles, we can use this fact to find missing angles.

Let's look at an example.

Find the missing angles  E and F

Angle E is vertically opposite to 40o

Therefore, E = 40o

Angle F forms a straight line with 40o

Angles on a straight line add up to 180o

Therefore, F = 180 - 40

F = 140o

Let's try some of these!

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