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G1a – 2D geometry – terms and notation
G1e – Conventions for labelling the sides and angles of triangles
G14a – Converting between metric units of measures of length and mass
G1c – Reflection symmetry and G1d – Rotation symmetry (required for the Challenge question only)

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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.

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Teachers: log in to access the following:

Slides in PPTX (with click-to-reveal answers)
Slides in PDF (one slide per page, suitable for importing into IWB software)

Worksheet (with space for student work)
Handout (slides with exercises only; 4 per page for reduced printing)
- Worked solutions to all questions (to ensure that the lengths on this answer sheet match the lengths on the printed worksheet, print on A4 paper using the “scale to fit” option)

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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.

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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. [/toggle] [toggle title=”Interactive GeoGebra activity: Standard protractor” state=”open”]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.