1.y = ax^2 + bx+c ,带入(1,-2),(0,3),(-1,-6)解得
y = -7x^2 + 2x + 3
2.y = a(x-3)^2 + b
b = 5
a(1-3)^2 + 5 = 11,a = 3/2
y = 3/2 (x-3)^2 +5
3.y = a(x-1+sqrt(2))(x-1-sqrt(2)) = a((x-1)^2 - 2) = a(x^2 - 2x - 1)
x = 0,y = -2
a = 2
y = 2(x^2-2x-1)