求解sin12sin48sin54的值
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sin12°sin48°sin54°
=（1/2）(cos36°-cos60°）cos36°
=(1/2)[(cos36°)^2-(1/2)cos36°]
=(1/4)[1+cos72°-cos36°]
=(1/4)[1-2sin54°sin18°]
=(1/4)[1-2sin54°sin18°cos18°/cos18°]
=(1/4)[1-cos36°sin36°/cos18°]
=(1/4)[1-sin72°/(2cos18°)]
=(1/4)(1-1/2)
=1/8.
=（1/2）(cos36°-cos60°）cos36°
=(1/2)[(cos36°)^2-(1/2)cos36°]
=(1/4)[1+cos72°-cos36°]
=(1/4)[1-2sin54°sin18°]
=(1/4)[1-2sin54°sin18°cos18°/cos18°]
=(1/4)[1-cos36°sin36°/cos18°]
=(1/4)[1-sin72°/(2cos18°)]
=(1/4)(1-1/2)
=1/8.
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