HOTS Zone : Barisan dan Deret Geometri

u₁ > u₂ maka |r| < 1.

\(\begin{aligned} u_5+u_7&=\dfrac{17u_6}4\\[4pt] ar^4+ar^6&=\dfrac{17ar^5}4&{\color{red}\times\dfrac1{ar^4}}\\[4pt] 1+r^2&=\dfrac{17r}4\\[4pt] 4+4r^2&=17r\\ 4r^2-17r+4&=0\\ (4r-1)(r-4)&=0 \end{aligned}\)
$r=\dfrac14$ atau r = 4 (TM)

\(\begin{aligned} u_2&=8\\ ar&=8\\ a\left(\dfrac14\right)&=8\\ a&=\color{blue}\boxed{\boxed{\color{black}32}} \end{aligned}\)