Chapter 11 - Further Topics in Algebra - 11.3 Geometric Sequences and Series - 11.3 Exercises - Page 1034: 29

$r=\dfrac {1}{5};a_{1}=125$

$\dfrac {a_{m}}{a_{n}}=r^{m-n}\Rightarrow \dfrac {a_{8}}{a_{3}}=r^{5}=\dfrac {\dfrac {1}{625}}{5}=\dfrac {1}{5^{5}}\Rightarrow r=\dfrac {1}{5};a_{3}=a_{1}\times r^{2}\Rightarrow 5=a_{1}\times \left( \dfrac {1}{5}\right) ^{2}\Rightarrow a_{1}=125$