C0+4⋅C1+7.C2+.(n+1) terms =
3n+22n-1
2n+33n-1
2n+32n-1
3n+23n-1
(n+1) th term of 1,4,7,… is 1+n(3)=3n+1 . Sum =(3n+1+1)2n−1=(3n+2)2n−1