y=cos(x＋π/3)－sin(x＋π/3)
=√2[(√2/2)cos(x＋π/3)－(√2/2)sin(x＋π/3)]
=√2cos[(x＋π/3)＋(π/4)]
=√2cos(x＋7π/12)
这个函数的最大值是√2,最小值是－√2,周期是2π