Answer
$\frac{-1}{xa}$
Work Step by Step
$\frac{f(x)-f(a)}{x-a}=\frac{\frac{1}{x}-\frac{1}{a}}{x-a}=\frac{\frac{a-x}{xa}}{x-a}=\frac{a-x}{xa(x-a)}=\frac{-1}{xa}$
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