y＝sin(x－π/3)sinx
＝1/2[cos(x－π/3－x)－cos(x－π/3＋x)]
＝1/2[1/2－cos(2x－π/3)]
＝1/4－1/2cos(2x－π/3)
≤1/4－1/2×(－1)＝3/4