Find Missing Angles using Geometrical Rules

In this activity, we will be looking at angle problems involving all the rules we have learnt previously!

Let's recap what we know already!

Make a note of what you need here!

Angles around a point = 360^o

Angles in a straight line = 180^o

Angles in triangles = 180^o

Angles in polygons = 180 x (number of sides - 2)

Vertically opposite (X) angles are equal

In parallel lines - (Z) alternate angles are equal

In parallel lines - (F) corresponding angles are equal

In parallel lines - (C) co-interior angles add up to 180^o

Polygons - exterior angle = 360 ÷ number of sides

Polygons - interior angle = 180 - exterior angle

Typical geometry questions will have a mixture of these rules included!

Let's have a look at a typical question.

Example

Calculate the missing angle A.

Answer

Step 1 - Angles in a straight line with 126^o

180 - 126 = 54^o

Step 2 - The triangle is isosceles so both the base angles are the same

Step 3 - Angles in a triangle add up to 180^o

We now have two angles so we can find the third one

A = 180 - (54 + 54)

A = 180 - 108

A = 72^o

Let's have a go at some other questions now.