b4=6 b9=192 S5-? bn=b1*q^(n-1) b1*q^3=6 b1*q^8=192 b1=6/q^3 (6/q^3)*q^8=192 6q^5=192 q^5=32 q=2 b1=6/2^3=6/8=3/4 Sn=(b1(q^n -1))/(q-1) S5=(3/4*(2^5 -1))/(2-1)=93/4