化简 cos（3k+1π/3 +X）+cos（3k-1π/3-X)其中k属于整数
问题描述：
化简 cos（3k+1π/3 +X）+cos（3k-1π/3-X)其中k属于整数
答
cos[(3k+1)π/3+x]+cos[(3k-1)π/3-x]
= 2 cos½[(3k+1)π/3+x+(3k-1)π/3-x] * cos½[(3k+1)π/3+x-(3k-1)π/3+x]
= 2 coskπ * cos(x+π/3)
= coskπ * (cosx - √3 sinx)
k 为偶数时,原式 = cosx - √3 sinx
k 为奇数时,原式 = - cosx + √3 sinx