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+2a+2c=2x²-4x+4所以2a=22b=-42a+2c=4a=1,b=-2,c=1f(x)=x²-2x+1