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(a+b)=cosacosb-sinasinb=1/4
cos(a-b)=cosacosb+sinasinb=2/3
所以cosacosb=(1/4+2/3)/2=11/24
sinasinb=(2/3-1/4)/2=5/24
tana*tanb=(sinasinb)/(cosacosb)=(5/24)/(11/24)=5/11

cos(a+b)=cosacosb-sinasinb=1/4
cos(a-b)=cosacosb+sinasinb=2/3
cosacosb=(1/4+2/3)/2=11/24
sinasinb=(2/3-1/4)/2=5/24
tana*tanb=(sinasinb)/(cosacosb)=(5/24)/(11/24)=5/11

cos(π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11)
=[2sin(π/11)cos(π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11)]/2sin(π/11)
=[sin(2π/11)cos(2π/11)cos(3π/11)cos(4π/11)cos(5π/11)]/2sin(π/11)
=[sin(4π/11)cos(3π/11)cos(4π/11)cos(5π/11)]/4sin(π/11)
=[sin(8π/11)cos(3π/11)cos(5π/11)]/8sin(π/11)
=[sin(3π/11)cos(3π/11)cos(5π/11)]/8sin(π/11)
=[sin(6π/11)cos(5π/11)]/16sin(π/11)
=[sin(5π/11)cos(5π/11)]/16sin(π/11)
=sin(10π/11)]/32sin(π/11)
=1/32