Prove that, Cos20°+Cos100°+Cos140°== 0
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We know

Therefore
cos 20 + cos 100 = 2.cos 60 cos 40
= 2. (1/2).cos 40
= cos 40
Hence cos 20 + cos 100 + cos 140 =
cos 40 + cos 140 =
2.cos 90 cos 50 = 2.0.cos 50 = 0
Hope this helps.
Therefore
cos 20 + cos 100 = 2.cos 60 cos 40
= 2. (1/2).cos 40
= cos 40
Hence cos 20 + cos 100 + cos 140 =
cos 40 + cos 140 =
2.cos 90 cos 50 = 2.0.cos 50 = 0
Hope this helps.
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