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Prove that: cos 10° + cos 110° + cos 130° = 0.

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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