`a ) -5x . ( 3x + 2 ) = 4x - 1`

`-15x^2 - 10x - 4x + 1 = 0`

`-15x^2 - 14x + 1 = 0`

`-15x^2 - 15x + x + 1 = 0`

`- ( 15x^2 + 15x ) + ( x + 1 ) = 0`

`-15x ( x + 1 ) + ( x + 1 ) = 0`

`( x + 1 ) ( -15x + 1 ) = 0`

`=> x + 1 = 0` `hoặc` `-15x + 1 = 0`

`=> x = 0 - 1` `hoặc` `-15x = 0 - 1`

`=> x = -1` `hoặc` `-15x = - 1`

`=> x = - 1` `hoặc` `x = -1 : ( -15 )`

`=> x = -1` `hoặc` `x = 1/15`

Vậy `x in { -1 ; 1/15 }`

`b ) { x + 4 }/5 = 1 - { 3x + 2 }/10`

`{ x + 4 }/5 + { 3x +2 }/10 = 1`

`{ 2 ( x + 4 ) }/10 + { 3x + 2 }/10 = 1`

`{ 2 ( x +4 ) + 3x + 2 }/10 = 1`

`{ 2x + 8 + 3x + 2 }/10 = 1`

`{ 5x + 10 }/10 = 1`

`5x + 10 = 1 . 10`

`5x + 10 = 10`

`5x = 10 - 10`

`5x = 0`

`x = 0 : 5`

`x = 0`

Vậy `x = 0`

`color{reD}{Fenik}`