Question
$f(x)=\frac{7}{x}-3, x \in R, x \neq 0$

Answer

$
\begin{aligned}
& f ( x )=\frac{7}{x}-3 \\
& \therefore f ^{\prime}( x )=\frac{d}{d x}\left(\frac{7}{x}-3\right)=7\left(-\frac{1}{x^2}\right)-0 \\
& =-\frac{7}{x^2}<0 \text { for all } x \in R , x \neq 0 \\
& \therefore f ^{\prime}( x )<0 \text { for all } x \in R , \text { where } x \neq 0 .
\end{aligned}
$
$\therefore f$ is decreasing for all $x \in R$, where $x \neq 0$.

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