$$f \frac{ x }{ \sqrt{ 4- { x }^{ 2 } } } DX$$
$\frac{DXfx}{\sqrt{4-x^{2}}}$
$|x|<2$
$\frac{4DXf}{\left(4-x^{2}\right)^{\frac{3}{2}}}$
$|x|<2$
