y=√3sina＋cosa=2[(√3/2)sina＋(1/2)cosa]=2[sinacos(π/6)＋cosasin(π/6)]=2sin(a＋π/6)
因为：a∈(－π/2,π/2),则：a＋π/6∈(－π/3,2π/3),得：sin(a＋π/6)∈(－√3/2,1],即：
y∈(－√3,2]