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Measuring line segments and angles in geometric figuresThis 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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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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