MCQ
The function $\sin x - \cos x$ is increasing in the interval
  • A
    $\left[ {{{3\pi } \over 4},{{7\pi } \over 4}} \right]$
  • $\left[ {0,{{3\pi } \over 4}} \right)$
  • C
    $\left[ {{\pi \over 4},{{3\pi } \over 4}} \right]$
  • D
    None of these

Answer

Correct option: B.
$\left[ {0,{{3\pi } \over 4}} \right)$
b
(b) We have, $f'(x) = \cos x + \sin x$

Now $f(x)$ is increasing function of  $x$ , if

$f'(x) = \cos x + \sin x > 0$ or $\sqrt 2 \cos \left( {x - \frac{\pi }{4}} \right) > 0$

==>$0 \le x < \frac{{3\pi }}{4}i.e.\,\,\,f'(x) > 0$ in $\left[ {0,\frac{{3\pi }}{4}} \right)$.

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