Do you know what the angles in a triangle add up to? If you don’t, we can find out by adding a parallel line to the opposite vertex of a triangle and using things that we already now.

What about a quadrilateral? Can we work it out by comparing it with a triangle?

Do you know what an **exterior angle** of a triangle is? What can we say about the sum of an exterior angle and an interior angle on the vertex of a triangle?

**Exercise**

Let’s complete exercise 9C from pages 134 to 137 of the textbook:

The answers are below: