Ratio of three consecutive terms in the binomial expansion of (1+x)n+2 is
Cr−1n+2:Crn+2:Cr+1n+2::1:3:5
So,
⇒Cr−1n+2Crn+2=13
⇒rn−r+3=3
⇒n+3=4r...(1)
And,
Crn+2Cr+1n+2=35
⇒r+1n+2−r=35
⇒3n+6−3r=5r+5
⇒3n+1=8r...(2)
So, 3(4r−3)+1=8r
⇒12r−9+1=8r
⇒4r=8
⇒r=2
So, n=5
Hence, we have
(1+x)n+2=(1+x)7
Sum of consecutive terms is
C17+C27+C37
=7+21+35=63