MCQ
$\mathop {\lim }\limits_{x \to 0} \frac{{\sin x - x}}{{{x^3}}} = $
  • A
    $\frac{1}{3}$
  • B
    $ - \frac{1}{3}$
  • C
    $\frac{1}{6}$
  • $ - \frac{1}{6}$

Answer

Correct option: D.
$ - \frac{1}{6}$
d
(d) $\mathop {\lim }\limits_{x \to 0} \,\frac{{\sin x - x}}{{{x^3}}}$

Expand $sin\ x$, then

$ = \mathop {\lim }\limits_{x \to 0} \frac{{ - \frac{{{x^3}}}{{3\,!}} + \frac{{{x^5}}}{{5\,!}} - ...}}{{{x^3}}} = \mathop {\lim }\limits_{x \to 0} \left[ { - \frac{1}{{3\,!}} + \frac{{{x^2}}}{{5\,!}} - ...} \right] $

$= \frac{{ - 1}}{{3\,!}} = \frac{{ - 1}}{6}$.

Aliter : Apply  $L-$ Hospital’s rule.

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