\[\boxed{\text{497\ (497).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[1)\ a + c.\]
\[2)\ m - n.\]
\[3)\ (x + y) \cdot (x - y).\]
\[4)\ (x - y)^{2}.\]
\[5)\ x^{2} - y^{2}\text{.\ }\]