TS EAMCET · Maths · Trigonometric Ratios & Identities
\(\cos \left(\frac{\pi}{7}\right) \cos \left(\frac{2 \pi}{7}\right) \cos \left(\frac{4 \pi}{7}\right)=\)
- A \(\frac{-1}{8}\)
- B \(\frac{1}{8}\)
- C \(-\frac{3 \sqrt{3}}{8}\)
- D 1
Answer & Solution
Correct Answer
(A) \(\frac{-1}{8}\)
Step-by-step Solution
Detailed explanation
We have, \[ \cos \frac{\pi}{7} \cdot \cos \frac{2 \pi}{7} \cdot \cos \frac{4 \pi}{7} \] Let \(A=\frac{\pi}{7} \Rightarrow \pi=7 A\) \[ =\cos A \cos 2 A \cos 4 A=\cos A \cos 2 A \cos 2^2 A \] Now, we will use the formula…
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