S6-?
Sn=b1(1-q^n)/(1-q)
S3=b1(1-q^3)/(1-q)=111
q^3=4
q=∛4
b1(1-(∛4)^3)/(1-∛4)=111
b1(1-4)/(1-∛4)=111
b1=-37+37∛4
S6=(-37+37∛4)(1-(∛4)^6)/(1-(∛4))=-37(1-4^2)=555
S6-?
Sn=b1(1-q^n)/(1-q)
S3=b1(1-q^3)/(1-q)=111
q^3=4
q=∛4
b1(1-(∛4)^3)/(1-∛4)=111
b1(1-4)/(1-∛4)=111
b1=-37+37∛4
S6=(-37+37∛4)(1-(∛4)^6)/(1-(∛4))=-37(1-4^2)=555