cos(x-2/3π)-cosx=cosxcos2/3π+sinxsin2/3π-cosx=-1/2cosx+(根3)/2sinx-cosx
=(根3)/2sinx-3/2cosx
=(根3)*(sinxcosπ/3-sinπ/3cosx)
=(根3)sin(x-π/3)
先分解再合并,最后提取出根号3就得到了sin(x-π/3)分解后的式子