# G15a – Measuring line segments and angles in geometric figures

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Prerequisites

# Measuring line segments and angles in geometric figures

This lesson covers the definitions of acute, right, obtuse, straight and reflex angles, as well as how to use protractors and rulers.
Teacher resources

Teaching notes

#### A possible approach

If your students are struggling to position the protractor in the “standard” correct way, you may wish to get them to focus on positioning the protractor’s centre on the vertex, but not worry about lining up one of the rays with 0º on the protractor, e.g. as follows:

Now students can pick either the inner or outer scale—it doesn’t matter which, and find the angle by calculating the difference between the readings corresponding to the two rays. In the above example, students could work out either 140 – 45 = 95º, or 135 – 40 = 95º. More details on this approach can be found in this blog post.

#### An alternative approach

Using an unnumbered protractor (see GeoGebra applet below) can help students appreciate angles as a measure of turn between two rays, while avoiding the confusion about whether to use the inner or outer scale.

Interactive GeoGebra activity: Unnumbered protractor
With thanks to Anthony C.M. OR, whose Measuring Angles Using Protractor worksheet is available here. Material modified and embedded here under the CC-BY-SA 3.0 license.
Interactive GeoGebra activity: Standard protractor
Drag the green points to form any angle up to 360º. Then drag the centre of protractor to the correct place by holding the protractor anywhere apart from the red point. Once the centre is positioned correctly, use the red point to rotate the protractor. Read the correct angle from the protractor. Tick the box to see if your reading matches the correct answer.
With thanks to Anthony C.M. OR, whose Measuring Angles Using Protractor worksheet is available here. Material modified and embedded here under the CC-BY-SA 3.0 license.
In the real world