cos[(3k+1)/3*π+α)+cos[(3k-1)/3*π+α]=2cos(kπ+α)*cos(π/3)=cos(kπ+α)
1、当k为偶数时,cos[(3k+1)/3*π+α)+cos[(3k-1)/3*π+α]=cos(kπ+α)=cosα
2、当k为奇数时,cos[(3k+1)/3*π+α)+cos[(3k-1)/3*π+α]=cos(kπ+α)=-cosα