`# \text {DNamNgV}`
`a)`
`(2x + 1)(3x - 2) + 9x^2 = 4`
`<=> 2x(3x - 2) + 3x - 2 + 9x^2 = 4`
`<=> 6x^2 - 4x + 3x - 2 + 9x^2 = 4`
`<=> 15x^2 - x - 2 = 4`
`<=> 15x^2 - x - 2 - 4 = 0`
`<=> 15x^2 - x - 6 = 0`
`<=> 15x^2 + 9x - 10x - 6 = 0`
`<=> (15x^2 + 9x) - (10x + 6) = 0`
`<=> 3x(5x + 3) - 2(5x + 3) = 0`
`<=> (3x - 2)(5x + 3) = 0`
`<=>` TH1: `3x - 2 = 0`
`<=> 3x = 2`
`<=> x = 2/3`
TH2: `5x + 3 = 0`
`<=> 5x = -3`
`<=> x = -3/5`
Vậy, `x \in {-3/5; 2/3}.`
`b)`
`x^3 - 4x = 0`
`<=> x(x^2 - 4) = 0`
`<=> x*(x^2 - 2^2) = 0`
`<=> x(x - 2)(x + 2) = 0`
`<=>` TH1: `x = 0`
TH2 `x - 2 = 0`
`<=> x = 2`
TH3: `x + 2 = 0`
`<=> x = -2`
Vậy, `x \in {+-2; 0}.`