$\displaystyle

(\sqrt{5}sin(\theta))^3\sqrt{5 - (\sqrt{5}sin(\theta))^2}\sqrt{5}cos(\theta)

$

$\displaystyle

= 25sin^3(\theta)(\sqrt{5cos^2(\theta)}\sqrt{5}cos(\ theta)

$

$\displaystyle

= 25sin^3(\theta)\sqrt{5}cos(\theta)\sqrt{5}cos(\the ta)

$

$\displaystyle

= 25sin^3(\theta)5cos^2(\theta)

$

$\displaystyle

= 125sin^3(\theta)cos^2(\theta)

$

this is slightly off from what the book has but it provides no explanation