The coefficient of 1x in the expansion of (1+x)n(1+1x)n is
The correct option is B (2n)!(n−1)!(n+1)!
Coefficient of x−1 in (1+x)n(1+1x)n
= Coefficient of x−1 in x−n(1+x)2n
= Coefficient of xn−1 in (1+x)2n
Now as we know that coefficient of xr in expansion of (1+x)n is nCr
Hence required coefficient is