a1= 1*2^1 a2=2*2^2 a3=3*2^3……an=n2^n
Sn=1*2^1 +2*2^2+ 3*2^3 + ……+n2^n
2*Sn=1*2^2 +2*2^3+ 3*2^4 + ……+(n-1)*2^n+n*2^(n+1)
2Sn-Sn=n*2^(n+1)-1*2^1 +1*2^2+ 1*2^3 + ……+2^n=n*2^(n+1)-(2-2^n*2)/(1-2)=(n-2)2^(n+1)+2