a1=1+1/2
a2=3+1/4
a3=5+1/8
a4=7+1/16
.
an=(2n-1)+1/2^n
Sn=[1+3+5+7+...+(2n-1)]+(1/2+1/4+1/8+...+1/2^n)
=(2n-1+1)*n/2+1-1/2^n
=n^2＋1-1/2^n