Câu b:
x^{2}+y^{2}-x^{2}y^{2}+xy-x-y
=(x^{2}-x^{2}y^{2})+(y^{2}-y)+(xy-x)
=x^{2}(1-y)(1+y)-y(1-y)-x(1-y)
=(1-y)(x^{2}+x^{2}y-x-y)
=(1-y)\left [ (x^{2}y-y)+\left (x^{2}-x \right ) \right ]
=(1-y)\left [ y(x-1)(x+1)+x(x-1) \right ]
=(1-y)(x-1)(xy+x+y)
Vậy...