Subject: Maths, asked on 16/11/12

20 points lie on a plane, out of which 7 points are collinear. Find the number of pentagons that can be drawn?

A :

15373

B :

15474

C :

15383

D :

12298

Time Spent: 1040 seconds

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A pentagon may be formed by joining 5 points lying on a plane. However, if we choose 3 or more points which are collinear, then the pentagon will not be formed.

Therefore, here we have 13 non-collinear points and 7 collinear points.

Number of pentagons that can be drawn= 13C5+ 13C47C1+ 13C37C2

= 1287 + 715 × 7 + 286 × 21 = 1287 + 5005 + 6006 = 12298

The correct answer is D.

how is it 13C47C1+ 13C37C2

pls can u explain why isit so?

Subject: Maths, asked on 26/9/15

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