Cos π/7+cos 6π/7+ sin π/5 - sin 4π/5
Cos π/7+cos 6π/7+ sin π/5 - sin 4π/5 = = cos π/7+cos(π-π/7)+sin (π/5)-sin (π-π/5) = =cos π/7 - cos π/7+sin π/5 - sin π/5 = 0