用向量法求下式的值:COS(2π/7)+COS(4π/7)+COS(6π/7)+COS(8π/7)+COS(10π/7)
化简sin(2π+a)cos(π-a)cos(π/2-a)cos(7π/2-a)/cos(π-a)sin(3π-a)si
已知cosα=1/7,cos(α+β)=-11/14,且α、β∈(0,2/Π),求cosβ的值
cos^2(π/6)
【紧急求助】计算:cosα+cos(2π/3 +α)+cos(4π/3 +α)
计算cos(a+5π/12)cos(a+π/6)+cos(π/12-a)cos(π/3-a)
化简cos(π/12)*cos(5π/12)
计算 sin5π/8*cos(7/8)π+sinπ/8-cos(3/8)π
已知α=7/12π,那么cosα√(1-sinα)/(1+sinα)+sinα√(1-cosα)/(1-cosα)=
不用计算器,求(cos(π/8))^4+(cos(3π/8))^4+(cos(5π/8))^4+(cos(7π/8))^
已知tan(π-α)=2,求sinα-2sinαcosα-cosα/4cosα-3sinα的值
求值:cos(25π /4)
cos(-π/2+a) 等于什么