prove cos x + cos 2x + cos 3x =0
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cos x + cos 2x + cos 3x = 0
=> cos 3x + cos x + cos 2x = 0
=> cos [(3x-x)/2] 2cos [(3x+3)/2] + cos 2x
=> 2cos 2x.cos x + cos 2x = 0
=> cos 2x [2cosx+1] = 0
=> cos 2x = 0
=> 2x = (2n+1) pi /2
n belongs to z
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