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sin54度%sin18度怎么化简 在线等急

和差化积公式 sin52-sin18=2cos36sin18 先乘cos18再除cos18 2cos36sin18cos18/cos18=cos36sin36/cos18=1/2sin72/cos18 sin72=cos18 则原式=1/2

cos36-cos72 =cos(54-18)-cos(54+18) =cos54cos18+sin54sin18-[cos54cos18-sin54sin18] =2sin54sin18 =-2sin54sin(-18) =2sin54sin18 =2cos36sin18 =2cos36sin18cos18/cos18 =cos36sin36/cos18 =sin72/(2cos18) =sin72/(2sin72)=1/2

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