Oh yes, the humble **triangle**, we take it for granted.

Its shape has been used by Egyptian kings, musicians and in warning signs, to name just a few of its many uses!

There are lots of **properties** (facts) about triangles that people have learned over the centuries, and now it is your turn.

** Special Triangles**

We need to look out for the markings on these triangles, as these quite often tell us which **type of triangle **we are looking at.

**e.g.** A triangle marked |||, || and | on the sides show that each side and angle is different, so therefore it must be **scalene**.

**e.g.** The two red angles in the** isosceles** triangle, highlight the base angles which are the same.

**A Key Triangle Fact:**

**Angles in a triangle always add up to 180°.**

Using this knowledge, we can find missing angles in a triangle.

Let's look at how we can do this now.

**e.g. What is the value of angle f in the first triangle below? And the value of angles a and b in the second triangle?**

From the markings on these triangles, we can see these are both **isosceles triangles**.

As a result, we know that the base angles must be **the same**.

**e.g. What is the value of angle b in the diagram below? **

Here, we have been asked to find an angle that is **external** to the triangle.

We need to put two rules we know together here - if we extend the base of the triangle we have a straight line.

What facts do we know about straight lines?

The angles on a straight line will also **add up to 180°**, so we can use this fact to find the missing exterior angle.

Right then, let's put what we know into action now.

In this activity, we will use the key fact, that angles in a triangle always add to 180°, to find the value of unknown angles in triangles, identify triangles accurately and solve problems involving triangles.