MA25-05 Maths Coming soon
Angle sum of a triangle and the exterior angle rule
This lesson is coming soon.
In this lesson
Use the fact that the angles in a triangle sum to 180 degrees and the exterior angle of a triangle equals the sum of the two interior opposite angles, including inside a chained isosceles-triangle problem.
What it covers
- Angle sum of a triangle = 180 degrees, including finding a missing angle
- Exterior angle of a triangle = sum of the two interior opposite angles (equivalently: exterior angle + adjacent interior angle = 180 degrees on a straight line)
- A chained isosceles-triangle example combining base-angles-are-equal with the 180-degree triangle sum or angles-on-a-line
Key words
For: Edexcel IGCSE 4MA1, AQA GCSE 8300, Edexcel GCSE 1MA1, Eduqas GCSE C300, Cambridge IGCSE 0580, OCR GCSE J560
On the specification
| Board | Spec | Statement |
|---|---|---|
| Edexcel IGCSE 4MA1 | F4.1C | Understand the exterior angle of a triangle property and the angle sum of a triangle property |
| AQA GCSE 8300 | G3 | Apply the properties of angles at a point, angles at a point on a straight line, vertically opposite angles |
| Edexcel GCSE 1MA1 | G3 | Apply the properties of angles at a point, angles at a point on a straight line, vertically opposite angles; understand and use alternate and corresponding angles on parallel lines; derive and use the sum of angles in a triangle (e.g. to deduce and use the angle sum in any polygon, and to derive properties of regular polygons) |
| Eduqas GCSE C300 | FG3 | Apply the properties of angles at a point, angles at a point on a straight line, vertically opposite angles; understand and use alternate and corresponding angles on parallel lines; derive and use the sum of angles in a triangle (e.g. to deduce and use the angle sum in any polygon, and to derive properties of regular polygons) |
| Eduqas GCSE C300 | HG3 | Apply the properties of angles at a point, angles at a point on a straight line, vertically opposite angles; understand and use alternate and corresponding angles on parallel lines; derive and use the sum of angles in a triangle (e.g. to deduce and use the angle sum in any polygon, and to derive properties of regular polygons) |
| Cambridge IGCSE 0580 | C4.6 | Angles |
| Cambridge IGCSE 0580 | E4.6 | Angles |
| OCR GCSE J560 | 8.03d | Derive and use the sum of the interior angles of a triangle is 180°. Derive and use the sum of the exterior angles of a polygon is 360°. Find the sum of the interior angles of a polygon. Find the interior angle of a regular polygon. |
For teachers
This GCSE Maths lesson teaches angle sum of a triangle and the exterior angle rule. By the end, students should be able to use the fact that the angles in a triangle sum to 180 degrees and the exterior angle of a triangle equals the sum of the two interior opposite angles, including inside a chained isosceles-triangle problem. It works through two worked examples and the mistakes examiners report.