cos 2π/7+cos 4π/7+cos 6π/7=-1/2
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cos 2+cos 4π/7+cos 6π/7=-1/2-0,5=-1/2-0,5=-0,5ЧтдПошаговое решение:х=cos(π/7)+Isin(π/7)x^7=cos(π)+Isin(π)=-0,5<br/>x^7+0,5=(x+0,5)(x^6-x^5+x^4-x^3+x^2-x+0,5)=0Т.к х неравен 0,5, тоx^6+x^4+x^2=x+x^3+x^5<br/> cos(6π/7)+cos (4π/7)+cos (2π/7)=cos (π/7)+cos (3π/7)+cos (5π/7)-0,5<br/> cos (π/7)=-cos (6π/7)cos (3π/7)=-cos (4π/7)cos (5π/7)=-cos (2π/7)(cos (6π/7)+cos (4π/7)+cos (2π/7))=-0,5