geometry · Year 6 · AC9M6M04
Angles on a straight line
A straight line is half a turn, so the angles that sit along one side of it add up to 180°. To find a missing angle, add the angles you know and take the total away from 180. The diagram is not drawn to scale, so work from the numbers, not by measuring. When two angles are marked with the same letter they are equal: share what is left between them.
Sample questions
The angles sit on a straight line. Find the angle marked with a question mark. The diagram is not drawn to scale.
- Angles on a straight line add up to 180°.
- 180 − 70 = 110, so the missing angle is 110°.
Answer: 110
The angles sit on a straight line. Find the angle marked with a question mark. The diagram is not drawn to scale.
- Angles on a straight line add up to 180°.
- 180 − 130 = 50, so the missing angle is 50°.
Answer: 50
The angles sit on a straight line. Find the angle marked with a question mark. The diagram is not drawn to scale.
- Angles on a straight line add up to 180°.
- 180 − 160 = 20, so the missing angle is 20°.
Answer: 20