#### Answer

$a_{5}=a_{1}\times r^{4}=10\times \left( -\dfrac {1}{2}\right) ^{4}=\dfrac {-5}{8};$
$a_{n}=a_{1}\times \left( r^{n-1}\right) =10\times \left( -\dfrac {1}{2}\right) ^{n-1}$

#### Work Step by Step

$r=\dfrac {\dfrac {-5}{4}}{\dfrac {5}{2}}=\dfrac {\dfrac {5}{2}}{-5}=\dfrac {-5}{10}=-\dfrac {1}{2};a_{1}=10\Rightarrow a_{5}=a_{1}\times r^{4}=10\times \left( -\dfrac {1}{2}\right) ^{4}=\dfrac {-5}{8};a_{n}=a_{1}\times \left( r^{n-1}\right) =10\times \left( -\dfrac {1}{2}\right) ^{n-1}$