（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
