`3^{2019}:(3^{2020}-24.3^{2017})`
`=3^{2019}:(3³ . 3^{2017} - 24 . 3^{2017})`
`= 3^{2019}:[3^{2017}. (3³ - 24)]`
`= 3^{2019}:[3^{2017} . 3]`
`= 3^{2019} : 3^{2018} = 3`
`.`
`5xy - 5x + y = 5`
`⇒ 5x(y - 1) + y - 1 = 5 - 1`
`⇒ 5x(y - 1 ) + (y - 1) = 4`
`⇒ (5x + 1)(y - 1) = 4`
`⇒ 5x + 1; y - 1 ∈ Ư(4) = {±1; ±2; ±4}`
$\text{Ta có bảng sau:}$
$\left[\begin{array}{ccc}5x+1&-4&-2&-1&1&2&4\\x&-1&/&/&0&/&/\\y-1&-1&-2&-4&4&2&1\\y&0&/&/&5&/&/\end{array}\right]$
$\text{Vậy (x; y) = (-1; 0), (0; 5)}$
