S6=3(a1+a6)=3(a2+a5)
36=3(3+a5)
a5=9
3d=a5-a2=9-3=6
d=2
a1=a2-d=1
an=a1+(n-1)d=1+2(n-1)=2n-1
an/2^n=(2n-1)/2^n
Tn=a1/2+a2/2^2+…+an/2^n
1/2Tn=a1/2^2+…+a(n-1)/2^n+an/2^(n+1)
用错位相减法求得Tn=3-(2n+3)/2^n