MCQ
${d \over {dx}}\left[ {\left( {{{{{\tan }^2}2x - {{\tan }^2}x} \over {1 - {{\tan }^2}2x{{\tan }^2}x}}} \right)\cot 3x} \right] =$
  • A
    $\tan 2x\,\tan x$
  • B
    $\tan 3x\tan x$
  • ${\sec ^2}x$
  • D
    $\sec x\tan x$

Answer

Correct option: C.
${\sec ^2}x$
c
(c) Let $y = \frac{{{{\tan }^2}2x - {{\tan }^2}x}}{{1 - {{\tan }^2}2x{{\tan }^2}x}}$

$= \frac{{(\tan 2x - \tan x)}}{{(1 + \tan 2x\tan x)}}\,\frac{{(\tan 2x + \tan x)}}{{(1 - \tan 2x\tan x)}}$

$= \tan (2x - x)\,\tan (2x + x)$$ =  \tan x\tan 3x$.

$\therefore$ $\frac{d}{{dx}}[y.\cot 3x] = \frac{d}{{dx}}[\tan x] = {\sec ^2}x$.

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