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