y=-1/2x^2+3x-1
=-1/2（x^2-6x）-1
=-1/2（x^2-6x+9-9）-1
=-1/2（x-3)^2+9/2-1
=-1/2（x-3)^2+7/2
顶点坐标为（3,7/2)
对称轴方程为x=3
开口向下