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 $\theta=$
- $\frac{\pi}{12}$
- $\frac{\pi}{4}$
- $\frac{\pi}{6}$
- $\frac{\pi}{3}$
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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