令f(x)=ax²+bx+c,则：
f(x+1)+f(x-1)
=a(x+1)²+b(x+1)+c+a(x-1)²+b(x-1)+c
=2ax²+2bx+2a²+2c
∴2a=2,2b=-4,2a²+2c=4
a=1,b=-2,c=1
f(x)=x²-2x+1