Naming angles
An angle shows the amount of turn between two lines that meet at a point. We measure the size of the turn in degrees (°), and we give each angle a name that tells us roughly how big it is. You can see angles all around you: the corner of a book, the hands of a clock, or a slice of pizza.
Key idea
Acute: less than 90°. Right: exactly 90°. Obtuse: between 90° and 180°. Straight: exactly 180°. Reflex: more than 180°. A full turn = 360°.
Worked example
Worked example
Two angles sit on a straight line. One is 120°. What is the other?
Angles on a straight line add up to 180°.
180 − 120 = 60°.
Check: 120 + 60 = 180°. Correct.
This is a very useful rule: whenever two angles sit side by side on a straight line, you can find one by subtracting the other from 180°.
Remember
- Right angle = 90°; straight angle = 180°.
- Angles on a straight line add up to 180°.
- Acute < 90° < obtuse < 180° < reflex.