prove that :
cos 24o + cos 55o + cos 125o + cos 204o + cos 300o = 
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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