Let the coefficients of rth,(r+1)th, and (r+2)th terms in the expansion of (1+x)n are nCr−1,nCr, and nCr+1 respectively. nCr−1:nCr:nCr+1=2:4:5 nCr−1nCr=
2
4
r
n−r+1
=
1
2
3r−1=n And,
nCr
nCr+1
=
4
5
r+1
n−r
=
4
5
5r+5=4n−4r 4n=9r+5 Solve and obtain r=3 and n=8..