\[\boxed{\text{671.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ a_{1} = 5,\ \ a_{2} = 10,\ \ q = 5:\]
\[x_{n} = 5 + 5 \cdot (n - 1) = 5 + 5n - 5 = 5n.\ \]
\[\textbf{б)}\ \ a_{1} = 6,\ \ a_{2} = 11,\ \ q = 5:\]
\[x_{n} = 6 + 5 \cdot (n - 1) = 6 + 5n - 5 = 5n + 1.\]