`a) 138 + x = 505`
`⇔ x = 505 - 138`
`⇔ x = 367`
Vậy `, x = 367 .`
`b) 28 - ( 2 . x - 5 ) : 3 = 7`
`⇔ ( 2 . x - 5 ) : 3 = 28 - 7`
`⇔ ( 2 . x - 5 ) : 3 = 21`
`⇔ 2 . x - 5 = 21 . 3`
`⇔ 2 . x - 5 = 63`
`⇔ 2 . x = 63 + 5`
`⇔ 2 . x = 68`
`⇔ x = 68 : 2`
`⇔ x = 34`
Vậy `, x = 34 .`
`c) 343 : ( 2^3 - 2^2 + 3^2 . x ) = 7`
`⇔ 2^3 - 2^2 + 3^2 . x = 343 : 7`
`⇔ 8 - 4 + 9 . x = 49`
`⇔ 4 + 9 . x = 49`
`⇔ 9 . x = 49 - 4`
`⇔ 9 . x = 45`
`⇔ x = 45 : 9`
`⇔ x = 5`
Vậy `, x = 5 .`
`d) 2^x + 2^( x + 1 ) = 24`
`⇔ 2^x . 1 + 2^x . 2 = 24`
`⇔ 2^x . ( 1 + 2 ) = 24`
`⇔ 2^x . 3 = 24`
`⇔ 2^x = 24 : 3`
`⇔ 2^x = 8`
`⇔ 2^x = 2^3`
`⇔ x = 3`
Vậy `, x = 3 .`
