cos(- 19/4 π)=?

cos(- 19/4 π)=? 化简cos(π/4-a)cos(π/4+a)= arccos(cosπ/4)=cos(arccosπ/4)对吗? cos(-19/6π)=? -cos(7/4) =cos(π-(7/4)) cos 2π/7 +cos 4π/7 +cos 6π/7=? 请问cos(2π/7)+cos(4π/7)+cos(6π/7)=?, cos(π/32)*cos(π/16)*cos(π/8)= 求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)(5/24)(11/24)=5/11?,而且过程和题好像都没关系。 cos^4(22.5)-cos^4(67.5)= 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^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? 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) 求证:cosαcos(π/3+α)cos(π/3-α)=cos3α如题,求证:4cosαcos(π/3+α)cos(π/3-α)=cos3α不好意思 少打个4 cos(-19π/4)的值