MCQ
The interval of increase of the function $\text{f}(\text{x})=\text{x}-\text{e}^{\text{x}}+\tan\Big(\frac{2\pi}{7}\Big)$ is:
  • A
    $(0,\infty)$
  • $(-\infty,0)$
  • C
    $(1,\infty)$
  • D
    $(-\infty,1)$

Answer

Correct option: B.
$(-\infty,0)$
$\text{f}(\text{x})=\text{x}-\text{e}^{\text{x}}+\tan\Big(\frac{2\pi}{7}\Big)$
$f^{\prime}(x)=1-e^x$
For $f(x)$ to be increasing, we must have
$f ^{\prime}(x)>0$
$\Rightarrow 1- e ^{ x }>0$
$\Rightarrow e ^{ x }<1$
$\Rightarrow x <0$
$\Rightarrow\text{x}\in(-\infty,0)$
So$, f(x)$ is increasing on $(-\infty,0).$

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