高一数学暑假作业
(2)、cos72-cos36=? (是度数)
cos72-cos36 (度全部省略) =2sin18sin54 (有这样一个和差化积公式cosA-cosB=-[sin(A+B)/2][sin(A-B)/2] =2sin18cos36 =2sin18cos18cos36/cos18 =sin36cos36/cos18 =(1/2)sin72/cos18 =1/2
cos72-cos36=cos(36+36)-cos36 =cos^36-sins^36-cos36 =cos^36-(1-cos36)^-cos36 =cos36-1
cos72-cos36=2(cos36)^2-1-cos36=(cos36-1)(2cos36+1)
答:分类讨论: n为偶数时,对边构成平行四边形,易证。 n为奇数时,取相邻两点弧的中点,构成偶数边 详见附件。。详情>>
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