-cos(7/4) =cos(π-(7/4))

cos(2派/7)+cos(4派/7)+cos(6派/7)=? 不用计算器,求(cos(π/8))^4+(cos(3π/8))^4+(cos(5π/8))^4+(cos(7π/8))^ cos(- 19/4 π)=? 已知cos（π4 已知sinθsinβ=-4/7,则cosθcosβ∈ 1：COS(-7π/12) = 已知(sinα-cosα)/(sinα+cosα)=1/3,则cos^4(π/3+α)-cos^4(π/6-α)的值为 sin^4+cos^4=1,求sin+cos= sina+cosa=7/13则5sina+4cos cos(α+β)=1/3,cos(α-β)=1/4,求cosα-cosβ, 求函数y=7-4sinxcosx+4cos^2x-4cos^4x的最大值与最小值 求函数Y=7-4sinxcosx+4cos^2X-4cos^4的最大值和最小值