cos36°-sin72°的值cos36°-cos - 知识问答

cos36°-sin72°的值cos36°-cos72°的值，抱歉题目打错了

3个回答

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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