Understand the idea.
Measure carefully
For an accurate measurement, first estimate whether the marked angle is acute, right or obtuse. Centre the protractor at the vertex, align zero with one ray and read the scale from that zero.
Look directly down at the scale and record the degree unit. Extend a short ray along the same straight line if needed. Finally compare your reading with the estimate to catch a wrong-scale answer.
Construct an angle
To construct an angle, draw a baseline with a marked endpoint as the vertex. Centre and align the protractor there, then mark the requested reading on the scale beginning at your baseline zero.
Remove the protractor and use a ruler to draw a ray from the vertex through the new mark. Label the angle and check the intended opening, rather than joining the mark to a different point.
See the method in action.
Construct an acute angle
Describe how to construct 45 degrees.
Worked solution
- 1
Draw a baseline from O.
The endpoint O will be the vertex shared by both rays.
- 2
Align the protractor at O.
Its centre must be on O and its zero line must follow the baseline.
- 3
Mark the 45-degree position.
Starting from the baseline zero places the mark halfway through a right-angle turn.
- 4
Join O to the mark and label.
The new ray and baseline form the requested acute opening; check it is half a right angle.
A correctly constructed 45-degree angle.
Construct an obtuse angle
Describe how to construct 120 degrees.
Worked solution
- 1
Draw a baseline and mark its vertex.
Both rays must start at the same point for the angle to be defined.
- 2
Centre and align the protractor.
The chosen scale must begin at zero on the baseline.
- 3
Count from zero and mark 120 degrees.
Use the 120 reading on that scale, even if the other scale shows 60.
- 4
Draw the second ray and check.
Joining the vertex to the mark gives an obtuse angle, as required.
A correctly constructed 120-degree angle.
Try it yourself.
Check your understanding
Should a 135-degree angle look acute or obtuse?
Show answer & reasoning
Obtuse
Since , the opening must be obtuse: wider than a right angle but smaller than a straight angle.
Keep practising
6 worksheet questions · 12 marks
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 ↗