（2X+b)^2= 4X ,4X^2 +4(b-1) X +b^2=0X1+X2= 1-b ,X1X2=b^2/4AB=2√5,直线y=2x+b 的 K=2 ,故X1-X2= 2故 X1-X2= √[(X1+X2)^2-4X1X2] =√[(1-b)²-b²]=√（1-2b)= 2即1-2b = 4 ,b= -3/2...