An **angle** is a measure of turn. We measure turn in **degrees**, where **360º is a complete turn**.

- An angle less than 90º is called an
**acute angle**; - an angle of exactly 90º is called a
**right angle**; - an angle of greater than 90º but less than 180º is called an
**obtuse angle**; - an angle of 180º is called a
**straight**angle (it is a straight line); and - an angle greater than 180º but less than 360º is called a
**reflex angle**.

We can use this information to identify the value of missing angles. Let’s try it out in the following short exercise (exercise 4 from pages 5 and 6 of the core textbook):

**Examples**: Let’s come up to the board individually to try some of these:

Calculate the size of the missing angles:

**Exercise**

Below are the answers:

**Triangles**

There are four important things that we should know about the angles in triangles:

- In all triangles, the three angles have a sum of 180º;
- In
**equilateral**triangles all of the angles are equal (so they are each 60º); - In
**isosceles**triangles, each of the two angles between unequal sides is equal; **The exterior angle of a triangle is equal to the sum of the two opposite interior angles.**

**Example: Find the missing angles**

**Exercise**: Let’s complete exercise 5 on pages 6 and 7 of the core textbook:

Below are the answers: