\[\boxed{\text{1068\ (1068).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{1} = x_{3} = 2a =\]
\[= (1 + ( - 1)^{n + 1}) \cdot a,\]
\[x_{2} = x_{4} = 2b = \left( 1 + ( - 1)^{n} \right) \cdot b,\]