Answer
$\frac{f(x)-f(1)}{x-1}=-\frac{1}{x+1}$
Work Step by Step
$\frac{f(x)-f(1)}{x-1}=\frac{\frac{x+3}{x+1}-\frac{1+3}{1+1}}{x-1}
=\frac{(x+3)-2(x+1)}{(x-1)(x+1)}
=\frac{x+3-2x-2}{(x-1)(x+1)}
=-\frac{x-1}{(x-1)(x+1)}
=-\frac{1}{x+1}$
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