JEE Mains · Maths · STD 11 - 3. trignometrical ratios,functions and identities
The value of \(\cos \left(\frac{2 \pi}{7}\right)+\cos \left(\frac{4 \pi}{7}\right)+\cos \left(\frac{6 \pi}{7}\right)\) is equal to
- A \(-\frac{1}{2}\)
- B \(-1\)
- C \(-\frac{1}{3}\)
- D \(-\frac{1}{4}\)
Answer & Solution
Correct Answer
(A) \(-\frac{1}{2}\)
Step-by-step Solution
Detailed explanation
\(\cos \frac{2 \pi}{7}+\cos \frac{4 \pi}{7}+\cos \frac{6 \pi}{7}\) \(=\frac{\sin \left(3 \times \frac{\pi}{7}\right)}{\sin \frac{\pi}{7}} \times \cos \left(\frac{\frac{2 \pi}{7}+\frac{6 \pi}{7}}{2}\right)\)…
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