The correct option is
A an(1+x)2n=1+2nC1x+2nC2x2+....+2nC2nx2nor,
(1+x)2n=a0+a1x+a2x2+...+a2nx2n ...(i)
Similarly,
(1−x)2n=a0−a1x+a2x2+...+a2nx2n ....(ii)
As, a0=a2n, a1=a2n−1,...and so on. So, (i) and (ii) can be written as:
(1−x)2n=a2n−a2n−1x+a2n−2x2+...+a0x2n
(1+x)2n=a0+a1x+a2x2+...+a2nx2n
So, required answer is coefficient of x2n in (1+x)2n.(1−x)2n= coefficient of x2n in (1−x2)2n
Tr+1=2nCr(−x2)r
=2nCr(−1)r(x)2r
So, we need, r=n. Hence, the answer is (−1)n.an
For n−even, the answer is option A, i.e; an