Question

hey guys
please help me with this question
I need a proper procedure with steps
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Answer 1

Step-by-step explanation:

Solution:-

We have,

LHS = cos A + cos (4/3 pi - A) + cos (4/3 pi + A)

= cos A + cos (240° - A) + cos (240° + A)

[ To convert into degree measure, multiply by 180°/pi]

= cos A + (cos 240°. cos A + sin 240°. sin A) + (cos 240°. cos A - sin 240°. sin A)

{Using : cos (x - y) = cos x. cos y + sin x. sin y and cos (x + y) = cos x. cos y - sin x. sin y}

On further solving,

cos A - 1/2 cos A - 1/2 cos A

{ Using cos 240° = - 1/2}

= cos A - cos A/ 2 - cos A/ 2

= 0

= RHS

Hence, proved.