For every value of $x$, the function $f(x)=\frac{1}{a^{x}}, a>0$ is

For every value of $x$, the function $f(x)=\frac{1}{a^{x}}, a>0$ is
  1. decreasing
  2. increasing
  3. Constant
  4. Neither increasing nor decreasing

Solution

$\begin{aligned} \mathrm{f}(\mathrm{x})=& \frac{1}{\mathrm{a}^{\mathrm{x}}}=\mathrm{a}^{-\mathrm{x}} \\ \therefore \quad \mathrm{f}^{\prime}(\mathrm{x}) &=-\mathrm{a}^{-\mathrm{x}} \cdot \log _{e} \mathrm{a}=-\frac{\log _{\mathrm{e}} \mathrm{a}}{\mathrm{a}^{\mathrm{x}}}= < 0 \end{aligned}$ So $\mathrm{f}(\mathrm{x})$ is decreasing for all $\mathrm{x}$.

Asked in: MHT CET 2020 (16 Oct Shift 2)

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