\[(x - y)^{2} - (x + y)^{2} = - 4xy\]
\[x^{2} - 2xy + y^{2} - x^{2} - 2xy + y^{2} = - 4xy\]
\[- 4xy = - 4xy\]
\[Что\ и\ требовалось\ доказать.\]