设解析式是y=a(x-2)(x+1)
x=0,y=-1代入得
-1=-2a
a=1/2
∴y=1/2(x-2)(x+1)
=1/2(x²+x-2x-1)
=1/2(x²-x-1)
=1/2(x²-x+¼)-5/8
=1/2(x-1/2)²-5/8
顶点坐标(1/2,-5/8)