cos[-(13π)/9]=cos[(5π)/9]= -cos(4π)/(9)[-(13π)/9]= -260度、[(5π)/9]=100度、(4π)/(9)=80度.负角的260度等于正角的100度我理解,但是等式最后一项“负的80度的余弦”我不太理解为什么要这么做.是因为要把

13sinα+5cosβ=9,13cosα+5cosβ=15,求sin(α+β)注意,是13cosα+5cosβ=15,不是13cosα+5sinβ=15 cos[-(13π)/9].[-(13π)/9]=-260度.但是为什么cos[-(13π)/9]不等于cos(-260度)? cos(π/12)cos(5π/12)= cos(5π/8)*cos（π/8)= cos^2 π/8+cos ^2 3π/8+ cos^2 5π/8+cos^2 7π/8=? cos(π/32)*cos(π/16)*cos(π/8)= 已知α为锐角,且cos(α+π/6)=5/13,则cosα= cos (180/5)* cos(360/5)=? 计算：cos（π/5)+cos（2π/5)+cos(3π/5)+cos(4π/5) cosπ/12= cos=-3/5, cos 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。 2cos9π/13*cosπ/13=[cos(9π/13+π/13)+cos(9π/13-π/13)]怎么化? 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) 已知sina=15/17,cosβ= - 5/13,a∈（π/2,π）,求cos（a+β）,cos（a-β）的值.急用