Primary 4 maths · Geometry · About 10 minutes
Use angle notation
Find the vertex and compare the opening, keeping the baseline fixed.
Start the interactive lesson →Open practice activity
What you will learn
- Use angle notation
Models used: number sentences, stories, diagrams.
Before you start
The middle letter names the corner
The two rays BA and BC meet at B. How do we name the angle between them?
Find the meeting point of the two rays. That point is B, the vertex of the angle.
Longer sides do not make a wider opening. A square corner is a useful reference; a protractor measures from the correct baseline.
Read from A along a ray to B, then along the other ray to C. Write ∠ABC; the vertex B stays in the middle.
∠ABC
Reading the other way gives ∠CBA. This names the same opening because B is still the middle letter.
∠ABC = ∠CBA
What did the picture help us see?
Reading the other way gives ∠CBA. This names the same opening because B is still the middle letter.
Worked example: A new example, together
Read the marked angle using the protractor.
Find the vertex and compare the opening, keeping the baseline fixed.
An angle measurement counts degrees from the correct baseline ray.
Use the zero scale aligned with the first ray. Centre the protractor at the vertex.
Use the zero scale aligned with the first ray. Read the marked angle using the protractor.
160 degrees
Centre the protractor at the vertex.
Use the zero scale aligned with the first ray. Read from zero on the first ray.
Use the zero scale aligned with the first ray. The second ray passes through 160°.
Use the zero scale aligned with the first ray. Check the result against the question we started with.
The supplementary angle is 20°; together they form 180°.
We found 160 degrees. An angle measurement counts degrees from the correct baseline ray.
Use the zero scale aligned with the first ray.
Common mistakes to discuss
- Naming the angle by the wrong letter. The middle letter is the vertex. ∠ABC is the angle at B, between the arms BA and BC.
Check your understanding
- Use angle notation
- recognise or measure a turn independently of ray length
- check the answer against the original quantities and units
Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.
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