(a-1)(a+1)(a²+1)
=(a²-1)(a²+1)
=a^4-1
(-3ab²+2a²b)(3ab²+2a²b)
=4a^4b²-9a²b^4
(1-xy)(-xy-1)
=-(1-xy)(1+xy)
=-(1-x²y²)
=x²y²-1