cos(α-55)cos(α+5)+sin(α-55)sin（α+5）化简

cosα 求证：|cosα+cosβ| 5cos^2a+4cos^2β=4cosα则cos^2a+cos^2β 求和：cos(0)+cos(α)+cos(2α)+cos(3α)+……cos(nα) 5cos^2α=1 cosα=? cos(α+5π/12)cos(α+π/6)+cos(π/12-α)cos(π/3-α) 化简 若sinα+cosβ=2/3 cosα+cosβ=5/6求cos(α-β) [sin(5π-α)cos(-π-α)]/[cos(α-π)cos(π/2+α)]化简 cos(α-55)cos(α+5)+sin(α-55)sin（α+5）化简 化简:cos(60-α)-cos(60+α) 已知5cos²α+4cos²β=4cosα则cos²α+cos²β范围 5cos^2 α +4cos^2 β =4cosα,求cos^2 α +cos^2 β的取值范围? 已知5(cosα)^2+4(cosβ)^2=4cosα,则(cosα)^2+(cosβ)^2的取值范围是? cosα*tanα sin(α+π/3)+sinα=负5分之4根号3 α∈（-π/2,0）求cosα怎样从9/4*sin²α=9/4(1-cos²α)=3/4*cos²a+12/5*cosx+48/25化为cos²α+4/5*cosα-11/100=0呀sin(α+π/3)+sinα=-4√3/5sinαcosπ/3+cosαsinπ/3+sinα=-4√3/53/2*si 若cosα 证明cos(α+β) 化简:cos(-π-α)