Ответ :
[tex] \displaystyle {p}^{2} {g}^{2} - {m}^{2} {n}^{2} = (pg - mn)(pg + mn); \: \\ \: \underline{\it \boxed{pravilo}: \: \: \: \star(a - b)(a + b) = { a}^{2} - {b}^{2} \star} \:[/tex]
[tex] \displaystyle {p}^{2} {g}^{2} - {m}^{2} {n}^{2} = (pg - mn)(pg + mn); \: \\ \: \underline{\it \boxed{pravilo}: \: \: \: \star(a - b)(a + b) = { a}^{2} - {b}^{2} \star} \:[/tex]