f(x)=ax^2+bx+c
代入
3=a-b+c (1)
3=a+b+c (2)
6=4a+2b+c (3)
(2)-(1)
0=2b,b=0
所以a+c=3 (4)
4a+c=6 (5)
(5)-(4)
3a=3
a=1
c=3-a=2
f(x)=x^2+2
f(x)=ax^2+bx+c
代入
3=a-b+c (1)
3=a+b+c (2)
6=4a+2b+c (3)
(2)-(1)
0=2b,b=0
所以a+c=3 (4)
4a+c=6 (5)
(5)-(4)
3a=3
a=1
c=3-a=2
f(x)=x^2+2