cos72度―cos36度=？，要过程。

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cos72―宽汪春陵辩cos

=cos(54+18)-cos(54-18)

=[cos54cos18-sin54sin18]-cos54cos18+sin54sin18
=-2sin54sin18
=-2cos36sin18
=-2cos36sin18cos18/cos18
=-cos36sin36/cos18
=-sin72/(2cos18)
=-sin72/(2sin72)
=-1/慎耐2

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