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