16(x+y)^2-25(x-y)^2
=[4(x+y)+(5(x-y)][4(x+y)-(5(x-y)]
=(4x+4y+5x-5y)(4x+4y-5x+5y)
=(9x-y)(9y-x)
(4x^2+y^2)^2-16x^2y^2
=[(4x^2+y^2)-4xy][(4x^2+y^2)+4xy]
=(2x-y)^2(2x+y)^2
16(x+y)^2-25(x-y)^2
=[4(x+y)+(5(x-y)][4(x+y)-(5(x-y)]
=(4x+4y+5x-5y)(4x+4y-5x+5y)
=(9x-y)(9y-x)
(4x^2+y^2)^2-16x^2y^2
=[(4x^2+y^2)-4xy][(4x^2+y^2)+4xy]
=(2x-y)^2(2x+y)^2