Precalculus (6th Edition) Blitzer

$\frac{1}{\sqrt {4+x}+2}$
Multiplying the conjugate of the numerator, we have $\frac{\sqrt {4+x}-2}{x}=\frac{\sqrt {4+x}-2}{x}\times\frac{\sqrt {4+x}+2}{\sqrt {4+x}+2}=\frac{4+x-4}{x(\sqrt {4+x}+2)}=\frac{1}{\sqrt {4+x}+2}$