`1) 4x(x-20) - (2x-1)(2x+3) = 0`
`<=> 4x^2 - 80x - (4x^2 + 4x - 3) = 0`
`<=> 4x^2 - 80x - 4x^2 - 4x + 3 = 0`
`<=> -84x + 3 = 0`
`<=> - 84x = -3`
`<=> x = 1/28`
Vậy `x = 1/28`
`2) (2x-1)^2 + (3+2x)(3-2x) = 8`
` <=> 4x^2 - 4x + 1 +(- 4x^2 + 9) = 8`
`<=> 4x^2 - 4x + 1 - 4x^2 + 9 = 8`
`<=> - 4x + 10 = 8`
`<=> - 4x = -2`
`<=> x = 1/2`
Vậy `x = 1/2`
`3) 3x^2 - 12x = 0`
`<=> 3x(x - 4) = 0`
`<=>` \(\left[ \begin{array}{l}3x=0\\x-4=0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=0\\x=4\end{array} \right.\)
Vậy `x∈{0;4}`
