Angles
Angles on a straight line
The angles that meet along a straight line always add up to 180°. You can use this to find a missing angle.
How to find a missing angle
1. Add up the angles you already know. 2. Subtract that total from 180°. 3. The answer is the missing angle.
Worked examples
Two angles on a line
- One angle is 105°.
- 180° − 105° = 75°.
Answer: The missing angle is 75°.
Three angles on a line
- The known angles are 55° and 80°.
- 55° + 80° = 135°.
- 180° − 135° = 45°.
Answer: The missing angle is 45°.
Where you see this in real life
- Working out the angle of a stage spotlight in a lighting plan.
- Finding the missing angle where slices of a semi-circular pie meet.
- Checking angles that sit along the edge of a ruler.
Key words
- Angle
- An amount of turn between two lines that meet at a point.
- Degrees (°)
- The unit of measurement for angles.
- Straight line
- The angles along a straight line add up to 180°.