求证cos2π/7+cosπ/3=cosπ/7+cos3π/7

求证cos2π/7+cosπ/3=cosπ/7+cos3π/7 cosπ/5×cos2π/5= cosπ/5cos2π/5=? cosπ/7cos2π/7cos3π/7=? cosπ/7cos2/7πcos4/7π= 若cos(π-θ)=3/5,cos2θ= cos(π-θ)=3/5 求cos2θ cos(π- θcos2θ)=3/5 求 已知cos(π/3-α)=根号3/3,求cos(2π/3+α)+cos2(7π/6+α) 化简cos2α+cos2(π/3+α)+cos2(π/3-α)化简cos²α+cos²（π/3+α）+cos²（π/3-α） 2sinα=sinθ+cosθ,sin²β==sinθcosθ.求证cos2β=2cos2α=2cos²﹙π＋θ﹚ 求证sin(α+β)+2cos(α-β)cosαcosβ=cos2α+cos2β 求值:cosπ/7*cos2π/7*cos3π/7 cosπ/9*cos2π/9*cos4π/9=? 求COSπ/5*COS2π/5 COSπ/5乘以COS2π/5 cosπ/5×cos2π/5 cosπ/5cos2π/5