cos a +cos (120-a)+cos (120+a)=0

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Solution

$T o p r o v e : \cos a + \cos (120 - a) + \cos (120 + a) = 0 P r o o f : u \sin g \cos (a + b) + \cos (a - b) = 2 \cos a \cos b = \cos a + 2 \cos (120) \cos a = \cos a + 2 \cos (90 + 30) \cos a = \cos a - 2 \sin (30) \cos a = \cos a - 2 (\frac{1}{2}) \cos a = \cos a - \cos a = 0$

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