Question
The function $\text{f}(\text{x})=\cot^{-1}\text{x}+\text{x}$ increases in the interval:
  1. $(1,\infty)$
  2. $(-1,\infty)$
  3. $(-\infty,\infty)$
  4. $(0,\infty)$

Answer

  1. $(-\infty,\infty)$
Solution:
$\text{f}(\text{x})=\cot^{-1}\text{x}+\text{x}$
$\text{f}'(\text{x})=\frac{-1}{1+\text{x}^2}+1$
f(x) is increasing,
$\Rightarrow\frac{-1}{1+\text{x}^2}+1>0$
$\Rightarrow\frac{\text{x}^2}{1+\text{x}^2}>0$
Hence, f(x) is increasing on $(-\infty,\infty).$

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