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  4. 1 + cos (π/7) + cos (2π/7)+ cos (3π/7)+ cos (4π/7)+ cos (5π/7)+ cos (6π/7)=

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Q. $1 + \cos \frac{\pi}{7} + \cos \frac{2\pi}{7}+\cos \frac{3\pi}{7}+\cos \frac{4\pi}{7}+\cos \frac{5\pi}{7}+\cos \frac{6\pi}{7}=$

Trigonometric Functions

Solution:

If $(A+ B) = \pi$ then $(\cos A + \cos B ) = 0$