`a, 217^50`
`= 217^(4 . 12 + 2)`
`= 217^(4 . 12) . 217^2`
`= (...1) . (...9)`
`= (...9)`
`b, 2021^2022 + 2022^2021`
`= 2021^(4 . 505 + 2) + 2022^(4 . 505 + 1)`
`= 2021^(4 . 505) . 2021^2 + 2022^(4 . 505) . 2022`
`= (...1) . (...1) + (...6) . 2022`
`= (...1) + (...2)`
`= (...3)`