设f（x）=ax^2+bx+c
那么f(x+1)+f(x-1)
=a(x+1)^2+b(x+1)+c+a(x-1)^2+b(x-1)+c
=2ax^2+2a^2+2bx+2c
=2x^2-4x
因为他们恒等于,那么
2ax^2=2x^2
2a^2+2c=0
2bx=-4x
解得a=1,b=-2,c=-1
所以f（x）=x^2-2x-1