#920 · Combinatorics · Counting

Number of triangles from points

With the same configuration of $p$ points (no three collinear except the $q$ collinear ones), find the number of distinct triangles whose vertices are among these points.

~5 minAnswer checked instantlytriangles from pointsexclude collinear triples

H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. XI. Permutations and Combinations, Examples XI.b, No. 24 (part 2)

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