Question

(viii) cos (120° + A) cos (120° - A) + cos (120° + A) cos A + cos A (cos 120° - A) +3/4=0
​

Answer 1

Answer:

(viii) cos (120° + A) cos (120° - A) + cos (120° + A) cos A + cos A (cos 120° - A) +3/4=0

Answer 2

Answer:

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