Đáp án:
`a, 4x(x + 1) + (3 - 2x)(3 + 2x) = 15`
`⇔ 4x^2 + 4x + 3^2 - (2x)^2 = 15`
`⇔ 4x^2 + 4x + 9 - 4x^2 = 15`
`⇔ (4x^2 - 4x^2) + 4x + 9 = 15`
`⇔ 4x + 9 = 15`
`⇔ 4x = 15 - 9 = 6`
`⇒ x = 6 : 4`
`⇒ x = 3/2`
Vậy `x = 3/2`
`b, 3x(x - 20012) - x + 20012 = 0`
`⇔ 3x(x - 20012) - (x - 20012) = 0`
`⇔ (x - 20012)(3x - 1) = 0`
`⇒` $\left[\begin{matrix} x - 20012 = 0\\ 3x - 1 = 0\end{matrix}\right.$
`⇒` $\left[\begin{matrix} x = 20012\\ 3x = 1\end{matrix}\right.$
`⇒` $\left[\begin{matrix} x = 20012\\ x = \dfrac{1}{3}\end{matrix}\right.$
Vậy `x = 20012` hoặc `x = 1/3`
