sn=2-2an
s(n-1)=2-2a(n-1)
an=Sn-S(n-1)=2a(n-1)-2an
3an=2a(n-1)
an/a(n-1)=2/3
a1=S1=2-2a1
a1=2/3
所以：an=(2/3)^n
Sn=2-2an=2-2*(2/3)^n
anSn=(2/3)^n*[2-2*(2/3)^n]
=2*(2/3)^n-2*(2/3)^2n
Tn=a1S1+a2S2+..+anSn
=2*[2/3+(2/3)^2+..+(2/3)^n]+2*[(2/3)^2+(2/3)^4+..+(2/3)^2n]
=2*Sn+2*(2/3)^2*[1-(2/3)^2n]/[1-(2/3)^2]
=4-4*(2/3)^n+8*[1-(2/3)^2n]/5