Hoặc
b) B=xx−1+3x+1−6x−4x−1
=xx+1x−1x+1+3x−1x−1x+1−6x−4x−1=x+xx−1+3x−3x−1−6x−4x−1=x+x+3x−3−6x+4x−1=x−2x+1x−1=x−12x−1x+1=x−1x+1