Triangle angle sum; the exterior angle theorem
- A triangle that adds to 270 — 3 tasks
- The two facts the proof runs on — 3 tasks
- Bringing the three angles to one point — 3 tasks
- What an exterior angle is equal to — 3 tasks
- Going further with the angle sum — 6 tasks
- Practice — 25 tasks
- Summary — 2 tasks
- Cool-down — 3 tasks
Classroom activities
Things a class does rather than writes on. Each one is a finished PDF: print it, cut it up if it says so, and hand it round.
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Remote Interior Angles — Cut & Glue Sort 25 minutes, ten of them cutting · 5 sheets
12 triangles with an angle drawn outside them and not a number anywhere: cut them out and sort them by whether the second marked angle is one of the two far ones, the one next to it, or neither — 4 to a bin, so a finished sheet is marked in ten seconds.
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Triangle Angle Sum & the Exterior Angle — Answer Maze 15–20 minutes · 4 sheets
One page and nothing to prepare: every box has a corner with one angle inside the triangle and one outside it, and asks for the one the exterior angle theorem is actually about — the row of box numbers at the foot marks the sheet in a glance.
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Exterior Angles of a Triangle — Mystery Picture 25–35 minutes · 4 sheets
25 triangles with an angle marked and not a number in sight: fill the square with slanted lines if the marked angle really is an exterior angle of that triangle, leave it white if it is not, and a picture appears.
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Triangle Angle Sum & the Exterior Angle — Scavenger Hunt a full class period · 13 sheets
16 cards round the room, each a triangle with one angle to find, and the card whose big number matches your answer is the next stop; get it wrong and you land on a card that names the mistake and sends you back, so a clean run is 8 lines on the record sheet.