Measuring angles (age 11+)
Draw and measure line segments and angles accurately using ruler and protractor, and interpret scale drawings to extract real measurements
What a learner can do afterwards
- Draw a triangle accurately given two sides and the included angle (SAS)
- Measure angles in a geometric figure to the nearest degree using a protractor
- Read a scale drawing to determine actual lengths, explaining the scale used
The lesson
You already know how to read a protractor and measure with a ruler. Now you will combine both skills: building a triangle exactly right, and reading real distances off a scale drawing.
To draw a triangle with a 5 cm side, a 7 cm side, and a 40 degree angle between them, start with the 5 cm side. At one end, use your protractor to mark a 40 degree angle. Draw the 7 cm side along that angle line, then join the two loose ends. The angle between the two given sides is called the included angle.
A scale drawing shrinks a real object down using a ratio, like 1 cm on the page standing for 2 m in real life. Measure the drawing with your ruler, then multiply by the scale number to find the real length.
Always round protractor readings to the nearest whole degree. Always multiply the ruler measurement by the scale number. Adding instead of multiplying is the most common scale mistake.
Match sides and the included angle exactly to construct a triangle, and multiply a scale drawing's measurement by its scale number to find the real length.
Watch it
Where it sits
Learn first
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.