Prove that : cos 20° + cos 100° + cos 140° = 0

Question

Philoid · Accepted Answer

Take L.H.S:

cos 20° + cos 100° + cos 140°

{cos (180° - A) = - cos A}

= 2 cos 60° cos (-40°) – cos 40°

{cos (-A) = cos A}

= cos 40° – cos 40°

= 0

= R.H.S

Hence Proved

Prove that :cos 20° + cos 100° + cos 140° = 0