和差化积公式cosα-cosβ=-2sin[(α+β)/2]·sin[(α-β)/2]推导过程：cos(a+b)=cosacosb-sinasinbcos(a-b)=cosacosb+sinasinb上面两式相减,得：cos(a+b)-cos(a-b)=-2sinasinb令a+b=xa-b=y则a=(x+y)/2b=...