`a_1) -2/5x + 1/5 = 3/4`
`⇒ -2/5x = 3/4 - 1/5`
`⇒ -2/5x = 11/20`
`⇒ x = 11/20 : -2/5`
`⇒ x = -11/8`
Vậy `x = -11/8`
`a_2) 2|2x - 1| - (-5)^2 = (-3)^0`
`⇒ 2|2x - 1| - 25 = 1`
`⇒ 2|2x - 1| = 1 + 25`
`⇒ 2|2x - 1| = 26`
`⇒ |2x - 1| = 26 : 2`
`⇒ |2x - 1| = 13`
`⇒ 2x - 1 = 13` hoặc `2x - 1 = -13`
`⇒ 2x = 13 + 1` hoặc `2x = -13 + 1`
`⇒ 2x = 14` hoặc `2x = -12`
`⇒ x = 14 : 2` hoặc `x = -12 : 2`
`⇒ x = 7` hoặc `x = -6`
Vậy `x ∈ {7; -6}`
`a_3) 3/4(x - 1) + 1 3/5 x - 30% = -2 4/15`
`⇒ 3/4 x - 3/4 + 8/5x - 3/10 = -34/15`
`⇒ 3/4x + 8/5x = -34/15 + 3/4 + 3/10`
`⇒ 47/20x = -73/60`
`⇒ x = -73/60 : 47/20`
`⇒ x = -73/141`
Vậy `x = -73/141`
`b)` Gọi `ƯCLN (2n + 5; 3n + 7) = d`
`⇒` $\left \{ {{2n + 5 \vdots d} \atop {3n+7\vdots d }} \right.$
`⇒` $\left \{ {{3. (2n+5) \vdots d} \atop {2.(3n+7)\vdots d }} \right.$
`⇒` $\left \{ {{6n+15\vdots d} \atop {6n+14\vdots d}} \right.$
`⇒ 6n + 15 - 6n - 14 vdots d`
`⇒ 1 vdots d`
`⇒ ƯCLN (2n + 5n; 3n + 7) = 1`
`⇒ (2n+5)/(3n+7)` là phân số tối giản.
