\frac{a}{b}:\frac{c}{d}= \frac{a}{b} \cdot \frac{d}{c} = \frac{ad}{bc},
\left( \frac{x}{y} \right)^{-z}= \left( \frac{y}{x} \right)^z,
 x^{-\frac{y}{z}} = (x^{-1})^{\frac{y}{z}} = \left(\frac{1}{x} \right)^{\frac{y}{z}},
\sqrt{a^2}= |a|