cos36cos72=1/4
cos12-cos24-cos48+cos84
=(cos12+cos84)-(cos24+cos48)
=2cos48cos36-2cos36cos12
=2cos36(cos48-cos12)
=2cos36×(-2sin30sin18)
=-2cos36sin18
=-2cos36cos72
=-2×1/4
=-1/2
已知cos36cos72=1/4求cos12-cos24-cos48+cos84
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