cos(2 兀/5)cos(兀/5-2)的值详细过程＿＿＿还有cosa=1/2其中a属于(-兀/2,

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应该是cos(2 π/5)cos(π/5-2π）
所以cos(2 π/5)cos(π/5-2π)
=cos(2 π/5)cos(π/5)
=[2sin(π/5)cos(π/5)cos(2π/5)]/[2sin(π/5)]
=sin(2π/5)cos(2π/5)/[2sin(π/5)]
=2sin(2π/5)cos(2π/5)/[4sin(π/5)]
=sin(4π/5)/[4sin(π/5)]
=sin(π/5)/[4sin(π/5)]
=1/4a∈(-π/2,0),∴a/2∈(-π/4,0),∴sin(a/2)
======以下答案可供参考======
供参考答案1:
不是楼上的有错是cos(2 兀/5)cos(兀/5-2)
供参考答案2:
cos(兀/5)cos(2兀/5)=2cos(兀/5)sin（π/5）cos(2兀/5)/2sinπ/5
= sin（2π/5）cos2π/5/2sinπ/5
=sin4π/5 / 4sinπ/5
=sinπ/5 / 4sinπ/5
=1/4-Pai/2cosa=1-2(sina/2)^2
1/2=1-2(sina/2)^2
(sina/2)^2=1/4
sina/2=-1/2.