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Questions  

If coefficient of 9th, 10th and 11th terms of (1 + x)n are in A.P., then value of n can be

a
15
b
32
c
23
d
17

detailed solution

Correct option is C

2 nC9=nC8+nC10⇒2=9n−8+n−910⇒20(n−8)=90+(n−8)(n−9)⇒n2−37n+322=0⇒(n−14)(n−23)=0Thus, n can be 23

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