向量2a=(2cosθ,2sinθ),
向量2a-b=(2cosθ-√3,2sinθ+1),
|2a-b}=√[(2cosθ-√3)^2+(2sinθ+1)^2]
=2√[2+2sin(θ- π/3）]
-1<=sin(θ- π/3)<=1,
∴0<=|2a-b|<=4,