积化和差公式：sinαsinβ=-[cos(α+β)-cos(α-β)]/2
f(x)=cos(x-π/3)-cos(π/3)
=cos(x-π/3)-1/2
<=1/2f(x)=cos(x-π/3)-cos(π/3)这步怎摸变得按照积化和差公式变得：sinαsinβ=-[cos(α+β)-cos(α-β)]/2