（1）设f(x)=ax^2+bx+c,
则,f（x+1）+f(x-1)
=a(x+1)^2+a(x-1)^2+b(x+1+x-1)+2c
=2(ax^2+bx+a+c)
=2x^2-4x
=2(x^2-2x)
所以,a=1,b=-2,a+c=0,c=-1,
f(x)=x^2-2x-1