`a,`
`(5x + 1)^{2} = 36/49`
`(5x + 1)^{2} = (+- 6/7)^{2}`
$\text{TH1:}$
`5x + 1 = 6/7`
`5x = 6/7 - 1`
`5x = -1/7`
`x = -1/7 : 5`
`x = -1/35`
$\text{TH2:}$
`5x + 1 = -6/7`
`5x = -6/7 - 1`
`5x = -13/7`
`x = -13/7 : 5`
`x = -13/35`
$\text{Vậy}$ `x \in {-1/35 ; -13/35}`
__________________
`b, (x + 1)^{3} = (-125)`
`(x + 1)^{3} = (-5)^{3}`
`x + 1 = -5`
`x = -5 - 1`
`x = -6`
$\text{Vậy}$ `x = -6`
