\[\boxed{\text{635.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{1} = 2,\ \ x_{4} = x_{1} \cdot q^{3} = \frac{1}{4},\ \ 2q^{3} = \frac{1}{4},\ \ q^{3} = \frac{1}{8},\]
\[q = \frac{1}{2},\ \ a = x_{2} = x_{1} \cdot q = 1,\ \ b = x_{3} = x_{2} \cdot q = \frac{1}{2}.\]
\[Ответ:a = 1,\ \ b = \frac{1}{2}.\]