[tex]\bf ~\hspace{7em}\textit{negative exponents} \\\\ a^{-n} \implies \cfrac{1}{a^n} ~\hspace{4.5em} a^n\implies \cfrac{1}{a^{-n}} ~\hspace{4.5em} \cfrac{a^n}{a^m}\implies a^na^{-m}\implies a^{n-m} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ \boxed{13}\qquad m^{-9}\cdot m^5\implies m^{-9+5}\implies m^{-4}\implies \cfrac{1}{m^4}[/tex]
[tex]\bf \boxed{15}\qquad 9n^{-3}\implies 9\cdot \cfrac{1}{n^3}\implies \cfrac{9}{n^3} \\\\\\ \boxed{17}\qquad b^3\cdot b^{-4}\cdot b^{-5}\implies b^{3-4-5}\implies b^{-6}\implies \cfrac{1}{b^6} \\\\\\ \boxed{18}\qquad \cfrac{4z^{-2}}{z^4}\implies \cfrac{4}{z^4z^2}\implies \cfrac{4}{z^{4+2}}\implies \cfrac{4}{z^6} \\\\\\ \boxed{20}\qquad \cfrac{18r^{-6}}{3r^3}\implies \cfrac{18}{3}\cdot \cfrac{r^{-6}}{r^3}\implies 6\cdot \cfrac{1}{r^3r^6}\implies \cfrac{6}{r^{3+6}}\implies \cfrac{6}{r^9}[/tex]
