Q: Given the integers r > 1, n > 2 , and coefficients of (3r)th and (r+2)nd term in the binomial expansion of (1+x)2n are equal, then
(A) n = 2r
(B) n =3r
(C) n = 2r+1
(D) None of these
Sol. Coefficients of (3r)th and (r+2)nd terms will be 2nC3r-1 and 2nCr+1
These are equal ⇒ (3r – 1) + (r + 1) = 2n ⇒ n = 2r
Hence (A) is the correct answer.