`4) (2x + 1)^2 = 16/25` 

`(2x + 1)^2 = (+-4/5)^2` 

`2x + 1 = +-4/5` 

`TH1: 2x + 1 = 4/5` 

`2x = 4/5 - 1` 

`2x = -1/5` 

`x = -1/10` 

`TH2: 2x + 1 = -4/5` 

`2x = -4/5 - 1` 

`2x = -9/5` 

`x = -9/10` 

`Vậy: x ∈ {-1/10; -9/10}` 

`------`

`5) (5x + 1)^2 = 36/49` 

`(5x + 1)^2 = (+-6/7)^2` 

`5x + 1 = +-6/7` 

`TH1: 5x + 1 = 6/7` 

`5x = 6/7 - 1` 

`5x = -1/7` 

`x = -1/35` 

`TH2: 5x + 1 = -6/7` 

`5x = -6/7 - 1` 

`5x = -13/7` 

`x = -13/35` 

`Vậy: x ∈ {-1/35; -13/35}` 

`------`

`6) (3x - 4)^2 = (-3/4)^2` 

`(3x - 4)^2 = (+-3/4)^2` 

`3x - 4 = +-3/4` 

`TH1: 3x - 4 = 3/4` 

`3x = 3/4 + 4` 

`3x = 19/4` 

`x = 19/12` 

`TH2: 3x - 4 = -3/4` 

`3x = -3/4 + 4` 

`3x = 13/4` 

`x = 13/12` 

`Vậy: x ∈ {19/12; 13/12}`