MCQ
$4 \tan ^{-1} \frac{1}{5}-\tan ^{-1} \frac{1}{70}+\tan ^{-1} \frac{1}{99}=$
  • A
    $\frac{\pi}{2}$
  • B
    $\frac{\pi}{3}$
  • $\frac{\pi}{4}$
  • D
    $\frac{\pi}{6}$

Answer

Correct option: C.
$\frac{\pi}{4}$
(C) $4 \tan ^{-1} \frac{1}{5}-\tan ^{-1} \frac{1}{70}+\tan ^{-1} \frac{1}{99}$
$=\tan ^{-1} \frac{120}{119}+\tan ^{-1} \frac{1}{99}-\tan ^{-1} \frac{1}{70}$
$=\tan ^{-1}\left(\frac{120}{119}\right)+\tan ^{-1}\left[\frac{\frac{1}{99}-\frac{1}{70}}{1+\frac{1}{99} \cdot \frac{1}{70}}\right]$
$=\tan ^{-1}\left(\frac{120}{119}\right)+\tan ^{-1}\left(\frac{-29}{6931}\right)$
$=\tan ^{-1} \frac{120}{119}-\tan ^{-1} \frac{29}{6931}$
$=\tan ^{-1} \frac{120}{119}-\tan ^{-1} \frac{1}{239}$
$=\tan ^{-1} 1=\frac{\pi}{4}$

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