Kinds of angles
A set square is a flat triangle in your pencil box. Its square corner is a right angle.
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Step 1 of 4
- Acute — less than a quarter turn.
- Right — exactly a quarter turn. Marked with a small square.
- Obtuse — more than a quarter turn, less than a half turn.
- Straight — exactly a half turn. One straight line.
The rule
- Acute — less than 90°.
- Right — exactly 90°. A book corner is one.
- Obtuse — more than 90° and less than 180°.
- Straight — exactly 180°. The arms make one line.
- Reflex — more than 180° and less than 360°.
A right angle is a quarter of 360°. So it is 90°. A straight angle is half of 360°. So it is 180°.
Example
An angle of 165° is obtuse. It is more than 90°. It is less than 180°.
An angle of 200° is reflex. It is more than 180°. It is less than 360°.