2cosx+1>=0
cosx>=-1/2=cos(2kπ±2π/3)
所以定义域的[2kπ-2π/3,2kπ+2π/3]cosx>=-1/2为什么会=cos(2kπ±2π/3)，求解释-1/2=cos(2kπ±2π/3)