Bài `2`: `a)`
`1 2/5 . x + 3/7 = -4/5`
`⇒ 1 2/5 . x = -43/35`
`⇒ x = -43/35 : 7/5`
`⇒ x = -43/49`.
`b)` `4/9 . x - 1/2 = 2/7`
`⇒ 4/9 . x = 11/14`
`⇒ x = 11/14 : 4/9`
`⇒ x = 99/56`
`c)` `1/2 . x - 3/4 = 1 1/4`
`⇒ 1/2 . x - 3/4 = 5/4`
`⇒ 1/2 . x = 2`
`⇒ x = 4`.
Bài `2`:
Ta có: `(x + 2)/11 + (x + 2)/12 + (x + 2)/13 = (x + 2)/14 + (x + 2)/15`
`⇒ (x + 2)/11 + (x + 2)/12 + (x + 2)/13 - (x + 2)/14 - (x + 2)/15 = 0`
`⇒ (x + 2) . (1/11 + 1/12 + 1/13 - 1/14 - 1/15) = 0`
Vì `1/11 + 1/12 + 1/13 - 1/14 - 1/15 \ne 0`
`⇒ x + 2 = 0`
`⇒ x = -2`