[tex]61= b_{1}q^4 \\
1647= b_{1} q^{10} \\ b_{7 }= b_{1}q^6 [/tex]
din prima relatie
[tex] b_{1}= \frac{61}{q^4} [/tex]
inlocuim in a doua relatie
[tex]1647= \frac{61}{q^4}* q^{10} \\ 27=q^6 \\ b_{1}= \frac{61}{ \sqrt{3} } \\ b_{7}= \frac{61}{ \sqrt{3} }*27= \frac{61 \sqrt{3} }{3}*27= 549 \sqrt{3} [/tex]