\[\boxed{\text{1222.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[2x^{2} + 2y^{2} = x^{2} + x^{2} + y^{2} +\]
\[+ y^{2} - 2xy + 2xy =\]
\[= \left( x^{2} - 2xy + y^{2} \right) +\]
\[+ \left( x^{2} + 2xy + y^{2} \right) =\]
\[= (x - y)^{2} + (x + y)^{2}\]