Trigonometry Crush 3

Geometry Level 3

cos ⁡ ( 2 π 7 ) + cos ⁡ ( 4 π 7 ) + cos ⁡ ( 6 π 7 ) = ? \large \cos\left(\frac{2\pi}7\right) + \cos\left(\frac{4\pi}7\right) + \cos\left(\frac{6\pi}7\right) = \ ?

− 1 2 -\frac12 1 2 \frac12 1 1 − 1 -1

Raj Rajput by RAJ RAJPUT , 25, India

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1 solution

Raj Rajput
Aug 11, 2015

11 Helpful 0 Interesting 0 Brilliant 0 Confused

Moderator note:

Yes this is a standard approach that doesn't require the use of complex numbers.

There's a much simpler approach. See this note by Chew-Seong Cheong.

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