SME
Grade 4 Math · Problem type
Measure angles with a protractor
Use this for “What is the measure of the angle?” on a protractor, either scale.
Diagram
The inner scale starts at 0 on the flat arm and reads 35°.
The outer scale reads 145°: 180° − 35°.
This is an example. Type your own values to start a new problem.
Angle
example · °
Reading on the other scale
calculated · °
Formulas
180 − 35 = 145
180 − angle = reading on the other scale
Assumptions
•
Put the center of the protractor on the corner and one arm along the 0° line.
•
Read the scale that starts at 0 on that arm. The other scale reads 180° minus the angle.
•
Angles from 0° to 180°.
Step-by-step
WE KNOW
Angle: 35°
FIND
reading on the other scale
Step 1 · Find reading on the other scale
180 − 35 = ?
180 − angle = reading on the other scale
The other scale counts from the other end of the protractor.
7
10
1
8
0
−
3
5
1
4
5
Reading on the other scale = 145°
Check
Put the numbers back in. Both sides should match.
180 − 35 = 145 ✓
Topic check
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