对称轴是x=-1.抛物线可表达为y = a(x + 1)² + d
x = 0,y = a + d = -1 (1)
x = 2,y = 9a + d = 1 (2)
(2)-(1):8a = 2,a = 1/4
d = -5/4
y = (x + 1)²/4 - 5/4 = x²/4 + x/2 - 1