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Question

Coefficient of 1x in the expansion of (1+x)n(1+1x)n is

A
n!(n1)!(n+1)!
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B
2n!(n1)!(n+1)!
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C
n!(2n1)!(2n+1)!
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D
2n!(2n1)!(2n+1)!
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Solution

The correct option is B 2n!(n1)!(n+1)!
This can be re-written as
(1+x)2nxn
Hence Tr+1=2nCrxrxn
Tr+1=2nCrxrn
Now for a term of 1x
rn=1
r=n1
Tn=2nCn1x1
Coefficient implies
2n!(n1)!(n+1)!

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