q^(7-4)=b7/b4=39/13=3, q^3=3,
b4=b1*q^3, b1=b4/q^3=13/3
b10=b1*q^9=13/3*27=117
(x-x+1)/(x-1) * y(x-1)/x= 1/(x-1) * y(x-1)/x= y/x