\[\boxed{\text{644.}\text{\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[x^{2} - 2xy + y^{2} = (x - y)^{2}\text{\ \ \ \ }\]
\[при\ x = 3 + \sqrt{5};y = 3 - \sqrt{5}:\]
\[(x - y)^{2} =\]
\[= \left( 3 + \sqrt{5} - 3 + \sqrt{5} \right) =\]
\[= \left( 2\sqrt{5} \right)^{2} = 4 \cdot 5 = 20.\ \]