cos(75°+α)=13
求证cos(720°+α)(2/cosα+tanα)(1/cosα-2tanα)=2cosα-3tanα
sinα+cosβ=sin225° sinα-cosβ=cos225°求cos(α-β)及cos(α+β)
cos²α+cos²(120°-α)+cosαcos(120°-α)化简
sinα+sinβ=sinγ cosα+cosβ=cosγ 证明cos(α-γ)
化简cosα+cos(120°-α)+cos(120+α)
已知3sinα-2cosα=0,求(cosα-sinα)/(cosα+sinα)+(cosα+sinα)/(cosα-s
求证:cos²α+cos²(α+β)-2cosαcosβcos(α+β)=sin²2β
已知cos(75°+α)=⅓,其中—180°<α<-90°,求sin(105°-α)+cos(375°-α)
xi=s*cos(α+β-180)
.[1/cos(-α)+cos(180°+ α )]/[1/sin(540°-α)+sin(360°-α)]=tan^3
cos(75°+α)=1/3 则cos(150°-α)等于几.急……