Question : If $\tan3\theta \tan7\theta = 1$, where $7\theta$ is an acute angle, then find the value of $\cot15\theta$.
Option 1: $1$
Option 2: $-1$
Option 3: $-\sqrt{3}$
Option 4: $\sqrt{3}$
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Correct Answer: $-1$
Solution :
$\tan3\theta \cdot \tan7\theta = 1$
⇒ $\tan 7\theta = \frac{1}{\tan 3\theta}$
⇒ $\tan7\theta = \cot3\theta$
⇒ $\tan7\theta = \tan(90° - 3\theta)$
⇒ $7\theta + 3\theta = 90°$
⇒ $10\theta = 90°$
⇒ $\theta = 9°$
$\therefore$ $\cot15\theta = \cot (15\times 9)°=\cot135° = \cot(90° + 45°)=-\tan45° = -1$
Hence, the correct answer is $-1$.
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