cos (120+A) cos (120-A)-sin(120+A)sin(120-A)
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Cos A + cos(120° + A) + cos(120 - A)
= Cos A + 2 cos{( 120°+A+120°-A)/2}cos{120°+A-120+A)/2 ………(1) by using the formula
Cos C + cos D = 2 cos (C+D)/2 cos (C-D)/2
(1) reduces to Cos A + 2 cos 120° cos A
= Cos A + 2 (-1/2) cos A
= 0
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