Acute, right, obtuse, straight — measure and calculate angles!
An angle is the amount of turn between two lines meeting at a point. We measure angles in degrees (°).
We use a protractor to measure angles.
| Name | Size | Example |
|---|---|---|
| Acute | Less than 90° | 45°, 30°, 75° |
| Right | Exactly 90° | 90° |
| Obtuse | Between 90° and 180° | 120°, 135° |
| Straight | Exactly 180° | 180° |
Angles that form a straight line add up to 180°.
Example: If one angle is 120°, the other is 180° − 120° = 60°.
Angles that meet at a point add up to 360°.
Example: Three angles are 90°, 120°, 80°. The fourth = 360° − 290° = 70°.
The three angles inside any triangle always add up to 180°.
Example: Two angles are 60° and 45°. Third = 180° − 60° − 45° = 75°.
What type of angle is 75°?
Two angles sit on a straight line. One is 75°. Find the other.
Three angles around a point: 90°, 120°, 80°. Find the fourth.
A triangle has angles 90° and 45°. Find the third.
An angle on a straight line is 140°. Find the missing angle.
Enter: 0 = acute, 1 = right, 2 = obtuse, 3 = straight
Find the missing angle on each straight line.
Find the 4th angle around each point.
Find the missing angle in each triangle.
Find each missing angle.