A) \[-1\]
B) \[0\]
C) \[1\]
D) None of these
Correct Answer: B
Solution :
\[2\,\cos \,\frac{\pi }{13}.\,\cos \frac{9\pi }{13}+\cos \frac{3\pi }{13}+\cos \frac{5\pi }{13}\] \[=2\,\cos \,\frac{\pi }{13}.\,\cos \frac{9\pi }{13}+2\cos \frac{4\pi }{13}+\cos \frac{\pi }{13}\] \[=2\,\cos \frac{\pi }{13}\left( \cos \,\frac{9\pi }{13}+\cos \frac{4\pi }{13} \right)\] \[=2\,\cos \frac{\pi }{13}\left( 2\cos \,\frac{\pi }{2}+\cos \frac{5\pi }{26} \right)=0\]
You need to login to perform this action.
You will be redirected in
3 sec
