Question: there are 12 points in a plane?
Answer: There are 12 points in a plane of which 5 are collinear. Find (i) the number of straight lines obtained by joining these points in pairs (ii) the number of triangles that can be formed with vertices at these points. ... of lines = 12C2 - 5C2 + 1 = 66 - 10 + 1 = 57.