(xy)^2-1
=(xy+1)(xy-1)
a^4x^2-a^4y^2
=a^4(x^2-y^2)
=a^4(x+y)(x-y)
3x^2+6xy+3y^ 2
=3(x^2+2xy+y^2)
=3(x+y)^2
(x-y)^2+4xy
=x^2-2xy+y^2+4xy
=x^2+2xy+y^2
=(x+y)^2
4a^2-3b(4a-3b)
=4a^2-12ab+9b^2
=(2a-3b)^2