If the coefficient of second, third and fourth terms in the expansion of (1+x)n  are in A.P., then n=  

Answers (1)

Coefficient of (r+1)th in the expansion of (1+x)n is nCr
Given 2nd, 3rd and 4th terms are in A.P
⇒2.nC2?=nC1?+nC3?
⇒n(n−1)=n+\frac{n(n-1)(n-2)}{6}
⇒6(n−1)=6+n2−3n+2
⇒n2−9n+14=0

⇒n=7

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