Power?

how do you do a negative power or minus a power

$\begin{array}{|rcll|} \hline a^{-n} &=& \dfrac{1}{a^n} \\\\ \dfrac{1}{a^{-n}} &=& a^n \\\\ \left( \dfrac{a}{b} \right)^{-n} &=& \left( \dfrac{b}{a} \right)^n \\ \hline \end{array}$