S=b1/(1-q)
S=3b1
3b1=b1/(1-q)
1-q=1/3
q=2/3
b2/b4=b1q/(b1q^3)=1/q^2=9/4
Sn=3b1
b1/1-q=3b1
q=1-1/3=2/3
b2/b4=b1q/b1q.^3
b2/b4=1/q.^2=
b2/b4=9/4