Teaching mathematics
Teaching mathematics

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Teaching mathematics

3.1 Using a protractor to measure angles

The half-circle protractor is commonly used to measure angles (Figure 9). The inner number scale rises from 0 to 180, from right to left, and the outer scale goes from 0 to 180, left to right. Learners commonly make mistakes because they are not using the correct scale when measuring the angle between two lines. The point to remember when measuring angles is that you need to start at zero on one of the lines and read round on that scale to see the number on the second line.

Described image
Figure 9 Using a protractor to measure an angle

In order to read the number from the protractor, it may help to use a ruler to make the lines longer so that they meet the scales on the protractor. Make sure that one of the lines in the angle sits on the zero line. Then, using the scale which starts at zero (the inner scale in this case), read off where the second line meets the scale (24⁰ for this particular angle). If a learner gets 156⁰ for the angle, it is obvious that they have made a mistake. Why is this obvious?

The angle is smaller than a right angle (acute), so the number of degrees must be less than 90.

Degrees in a circle

As you can see from the half-circle protractor displaying 180⁰, it is commonly accepted that there are 360⁰in a circle. No one is entirely sure where this originated, possibly with the Babylonians [2000-1600 BC]. It is certainly worth having at least one plausible explanation ready when teaching this in order to answer the question ‘Why?’


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371