## Precalculus (6th Edition)

$\dfrac{1}{r^3}$
RECALL: (1) $a^{-m} = \dfrac{1}{a^m}$ (2) $\dfrac{a^m}{a^n} = a^{m-n}$ (3) $\left(\dfrac{a}{b}\right)^m=\dfrac{a^m}{b^m}$ (4) $(ab)^m = a^mb^m$ Use rule (2) above to obtain: $\dfrac{r^{7}}{r^{10}}=r^{7-10} = r^{-3}$ Use rule (1) above to obtain: $=\dfrac{1}{r^3}$
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