-cos(7/4) =cos(π-(7/4))

-cos(7/4) =cos(π-(7/4)) cos 2π/7 +cos 4π/7 +cos 6π/7=? 请问cos(2π/7)+cos(4π/7)+cos(6π/7)=?, cos(2π/7)+cos(4π/7)+cos(6π/7)= -1/2.证明[cos(2π/7)]^2+[cos(4π/7)]^2+[cos(6π/7)]^2=5/4 cos(2π/7)+cos(4π/7)+cos(6π/7)= -1/2.证明[cos(2π/7)]^2+[cos(4π/7)]^2+[cos(6π/7)]^2=5/4 求数学高手帮忙cos(2π/7)+cos(4π/7))+cos(6π/7)=?(化简求值) cos(π/7) + cos(6π/7) =几 函数 (4 20:28:50)cos^4 (π /8)+cos^4(3π/8)+cos^4(5π/8)+cos^4(7π/8) = cos^2 π/8+cos ^2 3π/8+ cos^2 5π/8+cos^2 7π/8=? 化简求值cos(π/7)cos(2π/7)cos(4π/7) 求cos(2π/7)*cos(4π/7)*cos(6π/7)的值 cos (2 a/7）+cos(4 a/7)+cos(6 a/7)=? cos(2派/7)+cos(4派/7)+cos(6派/7)=? Cos[π/7] + Cos[2 π/7] + Cos[3 π/7] + Cos[4 π/7] + Cos[5 π/7] + Cos[6 π/7] + Cos[7π/7]过程思路 不要只有答案哦 cos（γ－π/3）=4√3/7,求cosγ cos(pai/15)×cos(2pai/15)×cos(3pai/15)×cos(4pai/15)×cos(5pai/15)×cos(6pai/15)×cos(7pai/15)求值 cos(pai/15)×cos(2pai/15)×cos(3pai/15)×cos(4pai/15)×cos(5pai/15)×cos(6pai/15)×cos(7pai/15)求值 已知sinθsinβ=-4/7,则cosθcosβ∈已知tan(π-θ)=3,求下式的值 (5xin^3θ+cosθ)/(2cos^3θ+sin^2θcosθ)