Question

Let the exact value of expression $E=\cos \frac{\pi }{7}+{\cos }^2\frac{\pi }{7}-2{\cos }^3\frac{\pi }{7}$ is a rational number in the form $p/q$. where $p$ and $q$ are natural numbers and co-prime, then $(p+q)=$

• A. 3
• B. 4
• C. 5
• D. 6

Answer 1

Answer: (C)

Correct answer is (c) 5