设f（x)=a(x-1)^2+b
f(1)=b=1
f(0)=a+b=-1
a=-2.b=1
f(x)=-2(x-1)^2+1
f(x)=0
（x-1)^2=1/2
x=1+√2/2
或
x=1-√2/2