y=-1/2x²+1/2x+1
y=-1/2(x²-x)+1
y=-1/2(x²-x+1/4)+1+1/2×1/4
y=-1/2(x-1/2)²+9/8
抛物线的对称轴是x=1/2,顶点坐标是（1/2,9/8),开口向下