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If  n1Cr=k23nCr+1 , then k belongs to 

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a
(−∞,−2]
b
[2,∞)
c
[−3,3]
d
(3,2]

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detailed solution

Correct option is D

We have,  n−1Cr=k2−3nCr+1⇒(n−1)!(n−1−r)!r!=k2−3n!(n−r−1)!(r+1)!⇒1=k2−3n(r+1) ⇒ r+1n=k2−3 [∵0≤r≤n−1→r+1≤n]⇒ k2−3≤1⇒k2−4≤0⇒−2≤k≤2                       (i)Again,  n−1Cr=k2−3nCr+1⇒ k2−3= n−1Cr nCr+1>0⇒ k2−3>0⇒k<−3 or k>3                          (ii)From (i) and (ii), we have k ∈ ( 3, 2]


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