Ta có: x−12x−13x−1
=2xx−1−1x−13x−1
=2x2−2x−x+13x−1
=2x2−3x+13x−1
=2x23x−1−3x3x−1+13x−1
=6x3−2x2−9x2+3x+3x−1=6x3−11x2+6x−1
