56. \(\dfrac{m+n}{m-n}-\dfrac{m-n}{m+n}\)

\[\frac{(m+n)^2-(m-n)^2}{(m-n)(m+n)}=\frac{m^2+2mn+n^2-(m^2-2mn+n^2)}{m^2-n^2}.\]

Ответ: \(\dfrac{4mn}{m^2-n^2}\), \(m\ne\pm n\).