Question

Ten points are plotted on a plane. Find the number of straight lines obtained by joining these points if four points are collinear

Answer 1

There are 10 points on a plane.

When 4 of them are collinear

∴ Number of lines passing through these points if 4 points are collinear

= ¹⁰C₂ – ⁴C₂ + 1

= `45 - (4!)/(2!2!) + 1`

= `45 - (4 xx 3 xx 2!)/(2 xx 2!) + 1`

= 45 – 6 + 1

= 40