$[0/1$ Points$]$

SPRECALC$71$

The first term of a geometric sequence is $25,$ and the fourth term is $\frac{1}{5}.$

$($a$)$ Find the common ratio $r.$

$r=$

Find the fifth term.

$a_5=$

$($b$)$ Find the partial sum of the first eight terms.

$S_8=$