geometry · Year 3 · AC9M3M05
Measuring and comparing angles
An angle measures the amount of turn between two arms, not how long the arms are. Two arms can be long or short — what matters is how far one is turned from the other. We measure the turn in degrees with a protractor. Line one arm up with the 0° mark and read the scale that starts from there. An acute angle is less than 90°, so its measurement must be a small number. Watch the two rows of numbers on a protractor: follow the same row you started from, or you will read the angle's supplement by mistake.
Sample questions
An acute angle has arms 5 cm long. On the protractor, its open arm points to the 55° mark, measured up from 0°. What is the size of the angle in degrees?
- Read the scale that starts at 0° on the same arm.
- The angle is acute, so it is less than 90°.
- The open arm reaches 55, so the angle is 55°.
Answer: 55
An acute angle has arms 3 cm long. On the protractor, its open arm points to the 58° mark, measured up from 0°. What is the size of the angle in degrees?
- Read the scale that starts at 0° on the same arm.
- The angle is acute, so it is less than 90°.
- The open arm reaches 58, so the angle is 58°.
Answer: 58
An acute angle has arms 8 cm long. On the protractor, its open arm points to the 52° mark, measured up from 0°. What is the size of the angle in degrees?
- Read the scale that starts at 0° on the same arm.
- The angle is acute, so it is less than 90°.
- The open arm reaches 52, so the angle is 52°.
Answer: 52