Building on angles on a straight line and vertically opposite angles students are taught why angles in a triangle have a sum of 180°. More able students are challenged to prove this using angles in parallel lines.

The start of the lesson recaps related angle properties such as those on a straight line and about a point. The main part introduces the sum of angles in a triangle as 180° and provides several examples for practice. The plenary challenges students to combine multiple properties within a single problem.

**Differentiated Learning Objectives**

- All students should be able to calculate angles in a scalene triangle.
- Most students should be able to calculate angles in an isosceles triangle and combine other angle properties.
- Some students should be able to prove angles in a triangle have a sum of 180°.