cos二分之三π＋α=cos(-2π+二分之三π＋α）=cos（α-π/2）=sin（π-α）=sinαcos（3π－α）=cos（π-α）=-cosα
cos(2π/3 +α)=cos(2π/3).cosα - sin(2π/3).sinα=-(1/2)cosα - (√3/2)sinα cos(3π +α)=cos(2π +(π +α))=cos(π +α)=-cosα
cosa -cosa
