Bài `1 : `
`a ) ƯCLN ( 18 ; 72 ) = 6`
`b ) ƯCLN ( 25 ; 75 ; 125 ) = 25 `
`c ) BCNN ( 12 ; 18 ; 21 ) = 126`
`d ) BCNN ( 9 ; 21 ; 15 ) = 215`
Bài `2 :`
`a ) 2 ( 2x - 5 ) = 10`
`4x - 10 = 10`
`4x = 10 + 10 = 20`
`x = 20 : 4 = 5`
Vậy `x = 5`
b ) `5x - 6 = 3x + 2 `
`2x = 8`
`x = 8 : 2 = 4`
Vậy `x = 4`
c ) `3 | 2x + 1 | = 15`
`| 2x + 1 | = 15 : 3`
$| 2x + 1 | = 5$
$⇒$\(\left[ \begin{array}{l}2x + 1 = 5⇔ 2x = 4 ⇔ x = 2 \\2x + 1 = -5 ⇔ 2x = -6⇔x = -3\end{array} \right.\)