y=2x^2-6x
=2(x^2-3x+9/4-9/4)
=2(x^2-3/2)^2+9/2
顶点坐标(3/2,9/2)对称轴x=3/2
y=3x^2-6x+5
=3(x^2-2x+1-1)+5
=3(x-1)^2+2
顶点坐标(1,2)对称轴x=1
y=-x^2/2-x+2
=-1/2*( x^2-2x+1+3)
=-1/2*(x-1)^2-3/2
顶点坐标(1,-3/2)对称轴x=1