📐 Angles
An angle is the amount of turn between two rays that share the same starting point. Let's explore the different types of angles and learn to measure them!
📐 Types of Angles
We measure angles in degrees (°). A full turn is 360°. Different angle sizes have different names.
Right Angle
Exactly 90°
Shown with a small square. Found at corners of squares and rectangles.
Acute Angle
Less than 90°
A small, sharp angle. Think: acute = cute and small!
Obtuse Angle
Between 90° and 180°
Wider than a right angle but not straight.
Straight Angle
Exactly 180°
A perfectly straight line. Half a full turn.
Revolution (Full Turn)
Exactly 360°
A complete rotation back to where you started.
🔢 Angle Size Order
Here is how the angle types compare in size:
📐 Using a Protractor
A protractor is a tool used to measure angles in degrees. It is shaped like a half-circle and has two scales (0–180°) running in opposite directions.
Steps to measure an angle:
- Place the centre point of the protractor on the vertex (corner) of the angle.
- Align the baseline (0° line) of the protractor along one arm of the angle.
- Read the scale where the other arm of the angle crosses the protractor.
- Check: is the angle acute or obtuse? Make sure your reading makes sense!
Tip: If the angle looks smaller than 90°, choose the reading that is less than 90°. If it looks bigger than 90°, choose the reading that is more than 90°.
Worked Examples
Example 1: Classify 45°
Is 45° acute, right, or obtuse?
45° is less than 90° — so it is acute.
45° is an acute angle. ✅
Example 2: Angles on a straight line
Two angles on a straight line must add up to 180°. If one angle is 65°, what is the other?
180° − 65° = 115°
The other angle is 115° (obtuse). ✅
Example 3: Angles in a right angle
A right angle is split into two parts. One part is 35°. What is the other part?
90° − 35° = 55°
The other part is 55° (also acute). ✅
Key Vocabulary
The point where two rays meet to form an angle.
The unit used to measure angles. A full turn = 360°.
A tool used to measure and draw angles.
A full turn of 360° — back to where you started.
Knowledge Check
Question 1: What type of angle is 120°?
Question 2: How many degrees is a right angle?
Question 3: Two angles on a straight line add up to 180°. One angle is 72°. What is the other?
Question 4: What tool do we use to measure angles?
Question 5: An angle of 27° is classified as:
Pax Says:
"Look around for angles at home! Door corners are right angles. An open book makes an obtuse angle. A slightly open door makes an acute angle. Geometry is everywhere!"
Key Concepts Summary
- ✓ Acute angle: less than 90°. Right angle: exactly 90°.
- ✓ Obtuse angle: between 90° and 180°. Straight angle: exactly 180°.
- ✓ Revolution: 360° — a full turn.
- ✓ Use a protractor to measure angles — place the centre at the vertex and align the baseline.