Similar triangles: equal angles and proportional sides
- Sorting five triangles — 3 tasks
- The statement is a machine — 3 tasks
- Where the two facts come from — 6 tasks
- Reading the proportion off the statement — 3 tasks
- Going Deeper — 4 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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Scale Factor, Inside One Triangle, or Neither? Similar Triangles — Cut & Glue Sort 25 minutes, including the cutting · 5 sheets
12 fractions of two sides, all from one drawing of two similar triangles meeting at a point. Students decide, without measuring, whether each fraction is the scale factor, a ratio inside one triangle, or neither (a side over a side of the other triangle that is not its partner).
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Which Way Up? Similar Triangles — Answer Maze 10–15 minutes · 4 sheets
8 boxes on one page, each a similarity statement with two sides of one triangle and one side of the other, and a missing side or a ratio to find. The wrong answers are the two costliest mistakes on this topic: using the scale factor upside down, and pairing the letters in ABC order instead of the order of the statement. The row of box numbers at the foot is the check.
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Always True? Similar Triangles — Mystery Picture 25–35 minutes · 4 sheets
25 squares, each a similarity statement and one equation, with no drawing and no number. Is the equation always true? Fill it or leave it white, and a picture appears.
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Which Side Is Its Partner? Similar Triangles — Scavenger Hunt a full class period · 13 sheets
16 cards around the room, each with two similar triangles side by side, nothing marked, and a similarity statement. Three sides of one triangle and one side of the other are given, and a missing side is asked. Matching sides by their place on the page lands on a card that sends the group back, and a clean run is 8 lines on the record sheet.