18 pts are indicated on the perimeter of a triangle ABC. How many triangles are there with vertices at these points?

note: each side contains 5 points in between the 2 vertices

Dear jee

total number of triangle = ¹⁸C3

but triangle can not be formed by joing the point which are in straight line

so possible triangle = ¹⁸C3 -3* ⁷C3

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