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在三角经常遇到证明下列恒等式 cos(2π/5)+cos(4π/5)=-1/2, cos(2π/7)+cos(4π/5)+cos(6π/7)=-1/2, cos(2π/9)+cos(4π/9)+cos(6π/9)+cos(8π/9) =-1/2,……或cos(π/5)+cos(3π/5)=1/2, cos(π/7)+cos(3π/7)+cos(5π/7)=1/2, cos(π/9)+cos(3π/9)+cos(5π/9)+cos(7π/9) =1/2, ……一般有 相似文献
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