2cos^2π/8-1=?

2cos^2π/8-1=? cos^2-π/8-1/2=? cos^2 π/8+cos ^2 3π/8+ cos^2 5π/8+cos^2 7π/8=? sinα+cosα=1,求cos^2α+cos^6α+cos^8α的值. 为什么cos(n-1)π=cos(n+1)πcos(n-1)π=cos(n-1+2)π=cos(n+1)π 对吗? 1-2cos^2(π/8)1-2cos^2(π/8)= cos(π/17)*cos(2π/17)*cos(4π/17)*cos(8π/17) 已知cos(π-a)*cos(3π/2+a)=-1/8,且π/4 cosπ/4 =2cos²π/8 -1这个是为什么 证明cos 2x cos x=1/2(cos x+cos 3x) cos^1度+cos^2度+cos^3度+……+cos^180度=? cos(π/3-α)=1/8,则cosα+（根号3）sinα的值为____已知cosα-cosβ=1/2,sinα-sinβ=1/3,求cos(α+β)的值 由cos(a+b)=cos a cos b-sin a sin b cos(a-b)=cos a cos b+sin a sin b解题设a为锐角,证：1、2分之根3乘cos a + 2分之1乘sin a=cos(6分之π-a)2、cos a-sin a=根号2cos(4分之π+a) cos^6(π/8)-sin^6(π/8)=求值,[cos^2(π/8)-sin^2(π/8)][cos^4(π/8)+sin^4(π/8)+cos^2(π/8)sin^2(π/8)]=cos(π/4)[1-cos^2(π/8)sin^2(π/8)]=cos(π/4)[1-[sin^2(π/4)]/4]==7√2/16 cos(2π/7)+cos(4π/7)+cos(6π/7)= -1/2.证明[cos(2π/7)]^2+[cos(4π/7)]^2+[cos(6π/7)]^2=5/4 cos(2π/7)+cos(4π/7)+cos(6π/7)= -1/2.证明[cos(2π/7)]^2+[cos(4π/7)]^2+[cos(6π/7)]^2=5/4 已知sin(3π+a)=1/4,求[cos(π+a)]/{cosa[cos(π+a)-1]}+[cos(a-2π)]/[cos(a+2π)cos(π+a)+cos(-a)]的值要具体过程 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)