[tex]y^4-2[/tex] is not a difference of squares: [tex]y^4[/tex] is the square of [tex]y^2[/tex], but 2 is not a perfect square
[tex]25m^2n^4-1[/tex] is a difference of squares: [tex]25m^2n^4[/tex] is the square of [tex]5mn^2[/tex] and 1 is the square of 1.
[tex]p^8-q^4[/tex] is a difference of squares: [tex]p^8[/tex] is the square of [tex]p^4[/tex] and [tex]q^4[/tex] is the square of [tex]q^2[/tex]
[tex]16x^2-24[/tex] is not a difference of squares: [tex]16x^2[/tex] is the square of [tex]4x[/tex], but 24 is not a perfect square.
