Which of the following is not a decreasing function on the interv… - Vidyadip

MCQ

Which of the following is not a decreasing function on the interval $\left( {0,{\pi \over 2}} \right)$

A
$\cos x$
B
$\cos 2x$
✓
$\cos 3x$
D
$\cot x$

✓

Answer

Correct option: C.

$\cos 3x$

c
(c) Obviously here $\cos 3x$ is not decreasing in $\left( {0,\frac{\pi }{2}} \right)$

because $\frac{d}{{dx}}\cos 3x = - 3\sin 3x$

But at $x = 75^\circ ,\,\, - 3\sin 3x > 0$.

Hence the result.

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