b4=8/27,q=1/3
bn=b1*q^(n-1)
b4=b1*q^3
b1*(1/3)^3=8/27
b1*(1/27)=8/27
b1=8
S=b1/(1-q)
S=8/(1-1/3)=8/(2/3)=12
b4=8/27,q=1/3
bn=b1*q^(n-1)
b4=b1*q^3
b1*(1/3)^3=8/27
b1*(1/27)=8/27
b1=8
S=b1/(1-q)
S=8/(1-1/3)=8/(2/3)=12