If 4 tan-1 x + cot‑1 x = π, then x equals:

  1. 1
  2. -1
  3. √3
  4. \(\dfrac{1}{\sqrt3}\)

Answer (Detailed Solution Below)

Option 4 : \(\dfrac{1}{\sqrt3}\)
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NDA 01/2025: English Subject Test
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Detailed Solution

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Concept:

\(\rm \cot \theta = \tan \left(\dfrac{\pi}{2}-\theta\right)\)​.

\(\tan^{-1} x = \dfrac{\pi}{2}-\cot^{-1} x\)

Calculation:

4 tan-1 x + cot‑1 x = π

\( 4\tan^{-1}x+\left(\dfrac{\pi}{2}-\tan^{-1}x\right)=\pi\)

\(3\tan^{-1}x=\pi-\dfrac{\pi}{2}=\dfrac{\pi}{2}\)

\( \tan^{-1}x=\dfrac{\pi}{6}\)

\(x=\tan\dfrac{\pi}{6}=\dfrac{1}{\sqrt3}\).

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