y=sin2x-根号3cos2x
=2[sin2xcosπ/3-cos2xsinπ/3]
=2sin(2x-π/3)
x∈[π/6,π/2]
当x=5π/12；y有最大值=2
当x=π/6,y有最小值=0
函数值域为[0,2]