MCQ
$\tan ^{-1}\left\{\frac{3 a ^2 x-x^3}{ a \left( a ^2-3 x^2\right)}\right\}=$
  • $3 \tan ^{-1} \frac{x}{a}$
  • B
    $2 \tan ^{-1} \frac{x}{a}$
  • C
    $\tan ^{-1} \frac{x}{a}$
  • D
    $\tan ^{-1} \frac{ a }{x}$

Answer

Correct option: A.
$3 \tan ^{-1} \frac{x}{a}$
(A) $\tan ^{-1}\left\{\frac{3 a ^2 x-x^3}{ a \left( a ^2-3 x^2\right)}\right\}=\tan ^{-1}\left\{\frac{3 a ^2 x-x^3}{ a ^3-3 ax ^2}\right\}$
$=\tan ^{-1}\left(\frac{3\left(\frac{x}{ a }\right)-\left(\frac{x}{ a }\right)^3}{1-3\left(\frac{x}{ a }\right)^2}\right)$
Put $\frac{x}{ a }=\tan \theta$
$\therefore \quad$ The given expression becomes
$\tan ^{-1}\left(\frac{3 \tan \theta-\tan ^3 \theta}{1-3 \tan ^2 \theta}\right)=\tan ^{-1}(\tan 3 \theta)$
$=3 \theta=3 \tan ^{-1} \frac{x}{ a }$

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