Remember that a difference of squares can be written as the product of two conjugate binomials:
[tex]a^2-b^2=(a+b)(a-b)[/tex]In the given expression, notice that m^4 and n^4 can be written as (m^2)^2 and (n^2)^2:
[tex]m^4-n^4=(m^2)^2-(n^2)^2[/tex]Once written as a difference of squares, we can factor the expression:
[tex](m^2)^2-(n^2)^2=(m^2+n^2)(m^2-n^2)[/tex]Therefore:
[tex]m^4-n^4=(m^2+n^2)(m^2-n^2)[/tex]