How many triangles can be obtained by joining 12 points, four of which are collinear?

Answer : Total number of points on plane = 12 Triangles can be formed from these points = ¹²C3

= 220

But 4 points are colinear, the number of triangles can be formed from these points

= ⁴C3

= 4

We need to subtract 4 from 220 because in the formation of triangles from 4 colinear points are added there.

So no of triangle formed is = 220 – 4

= 216