\[\boxed{\text{563\ (563).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ a_{100},a_{201},\ a_{n + 1},\ a_{n},\ a_{n + 2},\ a_{2n + 1};\]
\[\textbf{б)}\ \ a_{70},a_{99},\ a_{n - 3},\ a_{n + 2},\ a_{3n - 1}.\]