cos[﹙3k＋1／3﹚π＋α]＋cos[﹙3k－1／3﹚π－α]
=cos(3kπ)cos(π/3+a)-sin(3kπ)sin(π/3+a)+cos(3kπ)cos(π/3+a)+sin(3kπ)sin(π/3+a)
=2cos(3kπ)cos(π/3+a)
=2cos(kπ)cos(π/3+a)
=(-1)^k×2cos(π/3+a)
继续展开的话
=(-1)^k×2[cos(π/3)cos(a)+sin(π/3)sin(a)]
=(-1)^k[cos(a)+√3sin(a)]