cos36cos72如何化简求值

cos36-cos72如何化简求值 -
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 参考资料：还有呢？