Primary 4 Angles
Name an angle with the vertex in the middle, then measure it with a protractor.
Keywords
- Angle
- The turn between two rays that meet.
- Vertex
- The point where the two rays meet.
- Degree (°)
- The unit used to measure an angle.
- Protractor
- A tool for measuring angles in degrees.
Name an angle
Use three capital letters. The middle letter must be the vertex, where the two rays meet. Either endpoint can be written first.
For rays BA and BC, the vertex is B.
∠ABC = ∠CBA
Both names describe the same angle. The middle letter is B in each name.
Try together
In this 35° diagram, find the vertex and name the angle with three letters.
Reveal the answer
Find where the red rays meet. Put that letter in the middle. The vertex is B, so the angle is ∠ABC or ∠CBA.
Measure an angle
Put the centre mark on the vertex. Line up one ray with 0° and read the scale that starts at that 0°. An angle less than 90° is acute, 90° is right, and greater than 90° but less than 180° is obtuse.
Read the inner scale
Read the angle, then show the steps to check its measurement and name.
Look at the red rays. Show the first step when you are ready.
Read the outer scale
Start at the outer 0° mark on the left. Read the angle, then show the steps to check its measurement and name.
Look at the red rays. Show the first step when you are ready.