设f(x)=ax^2+bx+c
f(x+1)+f(x-1)=2a(x^2+1)+2bx+2c=2x^2-4x
对比系数：2a=2,2b=-4,2a+2c=0
a=1,b=-2,c=-1
f(x)=x^2-2x-1
f(1-√2)=1+2-2√2-2+2√2-1=0