π/6≤x≤2π/3 π/6-π/3≤x-π/3≤2π/3-π/3 -π/6≤x-π/3≤π/3, 画单位圆,看余弦线的变化趋势 1/2<=cos(x-π/3)<=1 1<=2cos(x-π/3)<=2 最大2 最小1