The idea, step by step
Measure carefully
Estimate the opening. Put the centre at the vertex and the zero line along a ray. Read where the other ray crosses the scale, then record degrees.
Look directly down at the scale. Extend a short ray with a ruler when necessary.
Construct an angle
Draw a baseline and mark a vertex at one end. Centre the protractor there and align zero with the baseline. Mark the requested degree position, remove the protractor and join the vertex to the mark.
Label your angle and check it against a benchmark. A 45-degree angle is half a right angle; 120 degrees is wider than a right angle.
A common trap
The 60 mark always constructs the requested 120-degree angle.
Instead: Which scale you use depends on the baseline. Start at its zero and count to 120.
See the thinking
Construct an acute angle
Describe how to construct 45 degrees.
- Draw a baseline from endpoint O.
- Centre the protractor at O and put zero on the baseline.
- Mark 45 and draw a ray from O through the mark. Check that it is acute.
Construct an obtuse angle
Describe how to construct 120 degrees.
- Draw a baseline and mark the vertex.
- Count from its baseline zero to 120.
- Mark that position and join it to the vertex. Check that the opening lies between 90 and 180 degrees.
Check your understanding
Should a 135-degree angle look acute or obtuse?
Show answer
Obtuse
135 is between 90 and 180 degrees.
Your turn
6 questions · 12 marks · Work at your own pace.
Read the questions online
- Construct a 45-degree angle with a ruler and protractor.2 marks
- Construct a 120-degree angle.2 marks
- Construct a 75-degree angle.2 marks
- Measure this angle.2 marks
- Measure this angle.2 marks
- A learner draws 80 degrees, then extends both rays without changing their direction. Does the angle change? Explain.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Construct a 45-degree angle with a ruler and protractor.2 marks
Draw baseline; align centre and zero; mark 45; join the vertex to the mark.
A 45-degree angle. - Construct a 120-degree angle.2 marks
Mark 120 from the correct zero; check that the opening is obtuse.
A 120-degree angle. - Construct a 75-degree angle.2 marks
Mark 75 from the baseline zero and draw the second ray.
A 75-degree angle. - Measure this angle.2 marks
Align the centre and right baseline, then read about 55.
About 55 degrees; allow roughly 2 degrees for careful printed measurement. - Measure this angle.2 marks
Use the right baseline zero and read about 155.
About 155 degrees; allow roughly 2 degrees for careful printed measurement. - A learner draws 80 degrees, then extends both rays without changing their direction. Does the angle change? Explain.2 marks
Ray length does not change the turn between the rays.
No; it stays 80 degrees.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Year 5 angle estimation and measurement using a protractor.
Official curriculum source ↗