cos A + cos (120° + A) + cos (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)/2cos(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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