an = Sn-S(n- 1)=(n+ 1)/2

2.bn=4^(n+ 1)/2 -4*(n+ 1)/2=2^(n+ 1)-2*(n+ 1)

Bn can be regarded as a composite of geometric progression 2 (n+ 1) and arithmetic progression -2*(n+ 1) (to be exact, the sum of them).

Find the first n items of the corresponding sequence and add them separately.