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
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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=5/4cos(2π/7)+cos(4π/7
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
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
根据二倍角公式,
[cos(2π/7)]^2=[cos(4π/7)+1]/2,
[cos(4π/7)]^2=[cos(8π/7)+1]/2=[cos(2π-8π/7)+1]/2=[cos(6π/7)+1]/2,
[cos(6π/7)]^2=[cos(12π/7)+1]/2=[cos(2π-12π/7)+1]/2=[cos(2π/7)+1]/2,
所以[cos(2π/7)]^2+[cos(4π/7)]^2+[cos(6π/7)]^2
=[cos(2π/7)+cos(4π/7)+cos(6π/7)+3]/2=[-1/2+3]/2=5/4
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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
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