Calculate Missing Angles in Triangles and Straight Lines

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 180^o

 

 

 

Rule 2

Angles in a triangle add up to 180^o

 

 

 

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

 

 

 

Answer

Angle E is vertically opposite to 40^o

Therefore, E = 40^o

 

Angle F forms a straight line with 40^o

 

 

Angles on a straight line add up to 180^o

Therefore, F = 180 - 40

F = 140^o

 

Let's try some of these!