1.y=2x²-4x-1
=2(x²-4x+4)-9
=2(x-2)²-9
所以开口向上,对称轴x=2,顶点(2,-9)
2.y=3x²-6x+2
=3x²-6x+3-1
=3(x-1)²-1
所以开口向上,对称轴x=1,顶点(1,-1)
3.y=(x+1)(x-2)
=x²-x-2
=x²-x+1/4-9/4
=(x-1/2)²-9/4
所以开口向上,对称轴x=1/2,顶点(1/2,-9/4)
4.y=-3(x+3)(x+9)
=-3(x²+12x+27)
=-3(x²+12x+36-9)
=-3(x+6)²+27
所以开口向下,对称轴x=-6,顶点(-6,27)