B=(x^2 -x +1) /(x^2 +x+1) =(x^2 +x +1-2x) /(x^2 +x+1)=1 -2x/(x^2 +x+1) =1+m
m =-2x/(x^2 +x+1)
m =0 => x =0 nhan m= 0 ; x khac 0 => m khac 0
m x^2 +(m+2)x+m=0
delta (x) >=0 <=>(m+2)^2 -4m^2 >=0 <=>-3m^2 + 4m+4>=0
delta (m) =4 + 12 =16
m1 =-2/3 ; m2 =2
2/3 <=m <=2
-1/3 <=B <=3