Let

$\displaystyle

x = 3\sin u

$

$\displaystyle

\int {\frac{{dx}}

{{x^2 \sqrt {9 - x^2 } }}} = \frac{1}

{9}\int {\csc ^2 udu} = - \frac{1}

{9}(\frac{{\sqrt {9 - x^2 } }}

{x}) + C

$

I understand everything until the end. Specifically,

how to get

$\displaystyle

(\frac{{\sqrt {9 - x^2 } }}

{x})

$

Any help would be greatly appreciated. Thanks.