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Prove that: cos 10° + cos 110° + cos 130° = 0. |
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Answer» L.H.S = cos 10° + cos 110° + cos 130° = (cos 10° + cos 110°) cos 130° = 2 cos (\(\frac{110+10°}{2}\)) cos ( \(\frac{110-10°}{2}\) ) + cos 130° [∵ cosC + cosD = 2 cos ( \(\frac{C+D}{2}\) ) cos ( \(\frac{C-D}{2}\))] = 2 cos 60° cos 50° + cos 130° = 2 × \(\frac{1}{2}\) cos 50° + cos (180° − 50) = cos 50° - cos 50° = 0 = R. H.S Hence Proved |
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