Question : Simplify the following:
$7\left(\cot 11^{\circ} \cot 13^{\circ} \cot 19^{\circ} \cot 29^{\circ} \cot 61^{\circ} \cot 71^{\circ} \cot 77^{\circ} \cot 79^{\circ}\right)$
Option 1: 7
Option 2: – 7
Option 3: – 1
Option 4: 1
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Correct Answer: 7
Solution :
$7\left(\cot 11^{\circ} \cot 13^{\circ} \cot 19^{\circ} \cot 29^{\circ} \cot 61^{\circ} \cot 71^{\circ} \cot 77^{\circ} \cot 79^{\circ}\right)$
$=7\left(\cot 11^{\circ} \cot 13^{\circ} \cot 19^{\circ} \cot 29^{\circ} \cot (90^{\circ}-29^{\circ}) \cot (90^\circ-19^\circ) \cot (90^\circ-13^\circ) \cot (90^\circ-11^\circ)\right)$
$=7\left(\cot 11^{\circ} \cot 13^{\circ} \cot 19^{\circ} \cot 29^{\circ} \tan 29^{\circ} \tan 19^{\circ} \tan 13^{\circ} \tan 11^{\circ}\right)$
We know that, $\cot\theta×\tan\theta=1$
$=7(1)$
$=7$
Hence, the correct answer is 7.
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