ab(a+1)(a-1)-ab
=ab(a^2-1-1)
=a^3b-2ab
[2(x^2y)^2-5x^3y+3x^2y^2]/x^2y
=2x^2y-5x+3y
(x^2-2y^2)^2-x^2(x+2y)(x-2y)
= (x^2-2y^2)^2-x^2(x^2-4y^2)
=x^4-4x^2y^2+4y^4-x^4+4x^2y^2
=4y^4