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**What is it?**

There are people (Mark McCourt, I am looking at you) who will say, quite rightly, that there is no such thing as a “Mastery Lesson”. Indeed, Mastery is a whole, long-term approach to mathematics that cannot be dipped in and out of from lesson to less, or week to week. However, there is little doubt in my mind that this lesson, whether you give it the Mastery label or not, is simply superb. It is all about angles on a straight line, and there are a number of features that I absolutely love:

The investigation at that start that cleverly removes the potential issue of students not being able to measure angles accurately

The use of an extra angle on the straight line that tackles a common misconception head-on and leads to a more accurate description of the rule than I have used in the past

The way the complexity builds until algebraic expressions are included alongside number-based examples

**How can it be used?**

As I say, this lesson will probably work best as part of a long-term approach to mathematics. Likewise, a good lesson for one teacher is not necessarily a good lesson for another – it needs to be carefully considered, adapted and tweaked to fit the needs of any given set of students. But I am convinced that there is enough in this lesson to significantly contribute to the development of a sound understanding of angles on a straight line. One thing I would add is a careful use of technology. Recreating some of the examples on GeoGebra may just help students visualise them more clearly and better understand how changes in the example lead to changes in the answers.

Thanks so much for sharing

Craig Barton

Download: Angles on a straight line – Mastery Lesson

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