f(x)=x²+px+q=x
x²+(p-1)x+q=0
x1=x2=2
p=-3,q=4
f(x)=x²-3x+4;
f(x-1)=x+1
(x-1)²-3(x-1)+4=x+1
整理得：x²-6x+7=0;
x=3±√2
B={3+√2,3-√2}