Answer :
FORMULA: -
(i) 
Proof:
⇒ cosx + cos (120 – x) + cos (120 + x) = cosx + 2cos120 cos( – x)
⇒ cosx + cos (120 – x) + cos (120 + x) = cosx + 2cos(180 – 60) cosx
⇒ cosx + cos (120 – x) + cos (120 + x) = cosx + 2 ( – cos 60) cosx
⇒ cosx + cos (120 – x) + cos (120 + x) = cosx – cosx
⇒ cosx + cos (120 – x) + cos (120 + x) = 0
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