y=ax²+bx+c
则
0=4a+2b+c (1)
-15/2=64a-8b+c (2)
3=a-b+c (3)
(2)-(1)
6a-b=-3/4 (4)
(1)-(3)
a+b=-1 (5)
(4)+(5)
7a=-7/4
a=-1/4
b=-3/4
c=5/2
y=-x²/4-3x/4+5/2