Indirect proof
- Certain without checking — 3 tasks
- Maya and the empty hall — 3 tasks
- Writing the temporary assumption — 3 tasks
- A triangle cannot have two right angles — 3 tasks
- Going Deeper — 5 tasks
- Practice — 24 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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Indirect Proof: Can Both Be True? — Cut & Glue Sort 25 minutes · 5 sheets
12 pieces, each two sentences about one figure. Students sort them into three rows: a definition forbids them together, the triangle angle sum forbids them, or they can both be true.
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Indirect Proof: Assume, Collide, Close — Maze 10–15 minutes · 4 sheets
8 boxes that walk through the three steps of an indirect proof: the assumption, the collision, and the last sentence. A wrong answer sends the student to a box that shows the mistake.
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Indirect Proof: Is the Assumption Correct? — Mystery Picture 25–35 minutes · 4 sheets
25 squares. Each gives a statement to prove and a temporary assumption, and asks one thing: is the assumption correct? The right answers draw a letter.