2cos^2(α+π/4) =2cos^2(α+π/4)-1+1 =cos[2(α+π/4)]+1 =cos(2a+π/2)+1 =1-sin2a怎么把cos变成sin的啊 用的是半角公式么?真的不懂啊

5cos^2a+4cos^2β=4cosα则cos^2a+cos^2β cos^2α+cos^2(α+π/3)-cosαcos(α+π/3)化简 计算：cos（π/5)+cos（2π/5)+cos(3π/5)+cos(4π/5) cos(π/17)*cos(2π/17)*cos(4π/17)*cos(8π/17) 【紧急求助】计算:cosα+cos(2π/3 +α)+cos（4π/3 +α） cos^2(π/4+α)=(cos(π/4)cosα-sin(π/4)sinα)² cos^2(π/4+α)是怎么等于(cos(π/4)cosα-sin(π/4)sinα)²的? sin(3π+α )=1/4,求cos(π+α)/cosα[cos(π+α)-1]+cos(α-2π)/cos(α+2π)cos(π+α)+coa(-α) cos 2π/7 +cos 4π/7 +cos 6π/7=? 请问cos(2π/7)+cos(4π/7)+cos(6π/7)=?, 求证cosα^2+cos(α+β)^2-2cosαcosβcos(α+β)=sinβ^2 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) cos(π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11) 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? 求和：cos(0)+cos(α)+cos(2α)+cos(3α)+……cos(nα) tan(π/4+α)=2,求1/(2cosαsinα+cos^2α) cos(α+β)=4/5cos(α-β)=-4/5,3π/2