cosx≥-1/2 = cos(-2π/3)=cos(2π/3)因为cosx在[0，π]上递减，在[π, 2π]上为递增函数，借助图像可得0<=x<=2π/3或4π/3<=x<=2π
cos≥-1/2cosx=-1/2x=2π/3x=4π/3所以【0,2π/3】和 【4π/3,2π】
