# How do you find the common ratio, r, for the geometric sequence that has a1 =100 and a8 = ?

Nov 8, 2015

$r = \sqrt[7]{{a}_{8} / {a}_{1}}$

See explanation.

#### Explanation:

The task is incomplete, because the term ${a}_{8}$ is missing, but it is possible to show the way to solve the task.

According to the $n - t h$ term formula we can write, that:

${a}_{8} = {a}_{1} \cdot {r}^{7}$, so

${r}^{7} = {a}_{8} / {a}_{1}$

$r = \sqrt[7]{{a}_{8} / {a}_{1}}$