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UKMT Senior Kangaroo, 2016

Combinatorics Difficulty 3.0 AMC 10/12 Prove it United Kingdom

Delia is joining three vertices of a square to make four right-angled triangles. She can create four triangles doing this, as shown. How many right-angled triangles can Delia make by joining three vertices of a regular polygon with 18 sides?

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