Measuring angles
Measure angles in whole-number degrees using a protractor; draw given angles and sketch angles of specified measure
What a learner can do afterwards
- Measure an angle with a protractor and read 47°
- Draw an angle of 135° using a protractor
- Identify which scale on the protractor to use for an obtuse angle
The lesson
A protractor is a curved tool marked with numbers from 0 to 180. To measure an angle, place the center point of the protractor exactly on the vertex, the corner where the two rays meet. Then line up the straight edge with one ray so that ray points to 0.
Look at an angle where one ray lines up with 0 and the other ray crosses the curved edge at the 47 mark. Read that number straight off the scale: the angle measures 47°.
A protractor has two rows of numbers, one counting up from the left and one counting up from the right, so each mark has two possible readings. Use what you already know about the angle to pick the right one: if it looks obtuse, choose the number between 90° and 180°. If it looks acute, choose the smaller number.
To draw a 135° angle, first draw a straight ray. Place the protractor's center on the end of that ray and line up 0 with the ray. Since 135° is obtuse, find the 135 mark using the row that runs between 90° and 180°. Mark a point there, then draw your second ray from the vertex through that point.
Line up the center on the vertex and one ray on 0, then read or mark the row of numbers that matches the size of angle you have.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.