若cosα=-3/5,π/2

若sinα+cosβ=2/3 cosα+cosβ=5/6求cos(α-β) 若sin(α-π)=2cos(2π-α),求（sinα+5cosα）/（sinα-3cosα）的值, 若(4sinα-2cosα)/(5cosα+3sinα)=10,且α∈(-π/2,0)则cosα值为 已知cos(α+β)=12/13,cos(2α+β)=3/5,若β∈(0,π/2),求cosα的值. cos^2 π/8+cos ^2 3π/8+ cos^2 5π/8+cos^2 7π/8=? 若cosα=-3/5,π/2 利用诱导公式求三角函数值 cos 6/65π一,利用诱导公式:cos（2kπ+α）=cosα （k∈Z）和cos（π－α）=－cosα 得出cos65π/6=cos(65π/6-10π)=cos(5π/6)=cos(π-π/6)=-cosπ/6=-√3/2二,又或者cos65π/6=cos(11π-1π/6)=-cos( 大人啊若(cosα-3/2π)=1/5,求cosα 速等啊若(cosα-3/2π)=1/5,求cosα 速等啊 若cos(α-5π)=-3/5 则cos(α+5π)=? cos^2α+cos^2(α+π/3)-cosαcos(α+π/3)化简 若cos(α+β)=4/5,cos(α+β)=-4/5,且π/2 若cos(π/2+α/2)=1/3,则cosα= cos(α+β)=4/5cos(α-β)=-4/5,3π/2 计算：cos（π/5)+cos（2π/5)+cos(3π/5)+cos(4π/5) 若2sinα+cosα=0,求(sinα)^2-3sinαcosα-5(cosα)^2除了用(sinα)^2+(cosα)^2=0解出sin,cos,还有什么方法解? 已知sinα+cosα=4/5且3π/22,若sinα+sinβ+sinγ=0,cosα+cosβ+cosγ=0,则cos(α-β)的值 求值:cos(π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11) 求值：cos(π/11)-cos(2π/11)+cos(3π/11)-cos(4π/11)+cos(5π/11)