Unit 1 · Lines and Angles
Copying and Bisecting Angles
Mathematics · Grade 7
What You Will Be Able To Do
📋Copy any angle onto a new ray without measuring it
✂️Cut any angle exactly in half, and into four or eight
🔺State the SSS condition and use it to justify both constructions
🔧Explain why a triangle is rigid and a four sided frame is not
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Three fixed lengths can be assembled into only one triangle. Because the shape is locked, the angles are locked too. So if you can force three sides to match, the angles match by themselves and you never have to measure them.
Presenter notes: Start by drawing an angle on the board and refusing to say what it is. Tell the class you want an exact copy of it on the far side of the board, and that nobody is allowed to measure anything. Let them suggest methods for a minute. Somebody will propose measuring, and you decline. Somebody may propose tracing, which is closer to the truth than they realise. Then say that the compass can do this, and that by the end of the lesson they will also know why it works, which is a single sentence about triangles.