For $\theta \in\left(0, \frac{\pi}{2}\right), \tan 3 \theta \cdot \tan 2 \theta \cdot \tan \theta+\tan 2…

For $\theta \in\left(0, \frac{\pi}{2}\right), \tan 3 \theta \cdot \tan 2 \theta \cdot \tan \theta+\tan 2 \theta+\tan \theta=1$, then $\theta=$
  1. $\frac{\pi}{12}$
  2. $\frac{\pi}{4}$
  3. $\frac{\pi}{6}$
  4. $\frac{\pi}{3}$

Solution

We have, $\tan 3 \theta=\tan (2 \theta+\theta)$ $\tan 3 \theta=\frac{\tan 2 \theta+\tan \theta}{1-\tan 2 \theta \tan \theta}$ $\therefore \tan 3 \theta-\tan 3 \theta \tan 2 \theta \tan \theta=\tan 2 \theta+\tan \theta$ $\therefore \tan 3 \theta \tan 2 \theta \tan \theta \quad=\tan 3 \theta-\tan 2 \theta-\tan \theta$ ...(1) We have $\tan 3 \theta \cdot \tan 2 \theta \tan \theta+\tan 2 \theta+\tan \theta=1$ $\therefore \tan 3 \theta-\tan 2 \theta-\tan \theta+\tan \theta+\tan 2 \theta=1 \Rightarrow \tan 3 \theta=1$ $\therefore \tan 3 \theta=\tan \frac{\pi}{4}=3 \theta=\frac{\pi}{4} \Rightarrow \theta=\frac{\pi}{12}$

Asked in: MHT CET 2020 (13 Oct Shift 2)

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