The idea, step by step
An angle measures a turn
Two rays meet at a vertex. The angle tells us how far one ray turns to reach the other. Longer rays do not make a wider angle: compare the opening, not the lengths of the lines.
A full turn is 360 degrees. A quarter turn is a 90-degree right angle. A half turn is a 180-degree straight angle.
Use benchmarks
An acute angle is greater than 0 but less than 90 degrees. An obtuse angle is greater than 90 but less than 180 degrees. A reflex angle is greater than 180 but less than 360 degrees.
Estimate before measuring. The arc tells you which of the two possible turns to consider.
A common trap
Longer rays must make a larger angle.
Instead: The turn between the rays determines angle size. Ray length does not.
See the thinking
A small turn
Classify 35 degrees.
- Compare it with a 90-degree right angle.
- 35 lies between 0 and 90, so the turn is acute.
Past a half turn
Classify 225 degrees.
- A straight angle is 180 degrees and a full turn is 360 degrees.
- 225 is between these benchmarks.
Check your understanding
Is exactly 90 degrees acute or right?
Show answer
Right
Acute angles are strictly smaller than 90 degrees.
Your turn
6 questions · 7 marks · Work at your own pace.
Read the questions online
- Classify 48 degrees.1 mark
- Classify 90 degrees.1 mark
- Classify 135 degrees.1 mark
- Classify 180 degrees.1 mark
- Classify 290 degrees.1 mark
- Two angles both measure 55 degrees. One has longer rays. Which is larger? Explain.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- Classify 48 degrees.1 mark
0 < 48 < 90.
Acute - Classify 90 degrees.1 mark
A quarter turn is exactly 90 degrees.
Right - Classify 135 degrees.1 mark
90 < 135 < 180.
Obtuse - Classify 180 degrees.1 mark
A half turn is 180 degrees.
Straight - Classify 290 degrees.1 mark
180 < 290 < 360.
Reflex - Two angles both measure 55 degrees. One has longer rays. Which is larger? Explain.2 marks
Both have the same 55-degree turn. Ray length does not change the angle.
Neither; they are equal.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Year 5 angle estimation and measurement using a protractor.
Official curriculum source ↗