Idea 8of 24Builds onPut the protractor on the vertex
Two scales: start at the 0
A protractor shows two numbers. Which one is the angle?
Today you'll discover
- why a protractor has two rings of numbers
- how to pick the right ring
- how an estimate catches the wrong one
Think firstThis arm crosses at 50 and at 130
This angle's first arm points left. Which number is its size?
Decide first, then press Next. Find the 0 that the first arm lies on.
A protractor on an angle whose first arm points left. Where the other arm crosses, two numbers are ringed: 50 on the outer ring and 130 on the inner ring. The rings are named, outer and inner, and the 0 under the first arm, on the outer ring, is ringed. A curved arrow counts up the outer ring from 0 to 50. The last step shows an angle whose first arm points right, crossing 50 and 130.
See the whole ideathe big idea and the key word, in one place
The big idea
Read the scale whose 0 sits on the angle's first arm.
Easy to get wrongThe most common slip is reading the wrong ring. Estimate first: an angle narrower than a square corner must read less than 90°.
Compare themOuter ring: 0 at the left, counting up towards the right. Inner ring: 0 at the right, counting up towards the left.
Key words
- inner scale
- the inner ring of numbers on a protractor, counting up from 0 at the right end
- outer scale
- the outer ring of numbers on a protractor, counting up from 0 at the left end
Ready to use it?
- Measure angles that open to the left and to the right.