f(x)=√（1-2cosx）
1-2cosx>=0
2cosx<=1
cosx<=1/2
π/3 +2kπ<=x<=5π/3 +2kπ ,k∈Z
即
[π/3 +2kπ,5π/3 +2kπ ],k∈Z
画图像
cosx=1/2
x=π/3或5π/3