Permutations and Combinations 2 Question 5

5. If nC4,nC5 and nC6 are in AP, then n can be

(2019 Main, 12 Jan II)

(a) 9

(b) 11

(c) 14

(d) 12

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Answer:

Correct Answer: 5. (c)

Solution:

  1. If nC4,nC5 and nC6 are in AP, then

2nC5=nC4+nC6

[If a,b,c are in AP, then 2b=a+c ]

2n!5!(n5)!=n!4!(n4)!+n!6!(n6)!

254!(n5)(n6)!

nCr=n!r!(nr)!

=14!(n4)(n5)(n6)!+1654!(n6)!

25(n5)=1(n4)(n5)+130

25(n5)=30+(n4)(n5)30(n4)(n5)

12(n4)=30+n29n+20

n221n+98=0

n214n7n+98=0

n(n14)7(n14)=0

(n7)(n14)=0

n=7 or 14



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