Coefficient of 1x in the expansion of (1+x)n(1+1x)n is
A
n!(n−1)!(n+1)!
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B
2n!(n−1)!(n+1)!
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C
n!(2n−1)!(2n+1)!
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D
2n!(2n−1)!(2n+1)!
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Solution
The correct option is B2n!(n−1)!(n+1)! This can be re-written as (1+x)2nxn Hence Tr+1=2nCrxrxn Tr+1=2nCrxr−n Now for a term of 1x r−n=−1 r=n−1 Tn=2nCn−1x−1 Coefficient implies 2n!(n−1)!(n+1)!