∵y＝cos(x－π/3)cos(x＋2π/3)＝cos(x－π/3)﹛﹣cos[π－(x＋2π/3)]﹜＝cos²(x－π/3)
＝1/2×[cos(2x－2π/3)＋1]＝1/2×[cos(2x－π/3)＋1]
∴最大值＝1