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Prove that : cos 24° + cos 55° + cos 125° + cos 204° + cos 300° = \(\cfrac12\)

Answer»

LHS  = cos 24° + cos 55° + cos 125° + cos 204° + cos 300°

= cos 24° + cos (90° × 1 – 35°) + cos (90° × 1 + 35°) + cos (90° × 2 + 24°) + cos (90° × 3 + 30°)

We know that when n is odd, cos → sin.

= cos 24° + sin 35° - sin 35° - cos 24° + sin 30°

= 0 + 0 + 1/2 = 1/2

= RHS

Hence proved.



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