Đáp án:
\(28)\left( {4x - 1} \right)\left( {2x + 3} \right)\)
Giải thích các bước giải:
\(\begin{array}{l}
18)5{x^2} - 20x + 2x - 8\\
= 5x\left( {x - 4} \right) + 2\left( {x - 4} \right)\\
= \left( {x - 4} \right)\left( {5x + 2} \right)\\
19)6{x^2} - 2x + 9x - 3\\
= 2x\left( {3x - 1} \right) + 3\left( {3x - 1} \right)\\
= \left( {3x - 1} \right)\left( {2x + 3} \right)\\
20)3\left( {{x^2} - 2x + x - 2} \right)\\
= 3\left[ {x\left( {x - 2} \right) + x - 2} \right]\\
= 3\left( {x - 2} \right)\left( {x + 1} \right)\\
21)3\left( {{x^2} + 2x - x - 2} \right)\\
= 3\left[ {x\left( {x + 2} \right) - \left( {x + 2} \right)} \right]\\
= 3\left( {x + 2} \right)\left( {x - 1} \right)\\
22)6{x^2} + 3x + 12x + 6\\
= 3x\left( {2x + 1} \right) + 6\left( {2x + 1} \right)\\
= \left( {2x + 1} \right)\left( {3x + 6} \right)\\
23)6{x^2} + 3x + 12x + 6\\
= 3x\left( {2x + 1} \right) + 6\left( {2x + 1} \right)\\
= \left( {2x + 1} \right)\left( {3x + 6} \right)\\
24)6{x^2} - 18x - 2x + 6\\
= 6x\left( {x - 3} \right) - 2\left( {x - 3} \right)\\
= \left( {x - 3} \right)\left( {6x - 2} \right)\\
25) - 8{x^2} + 8x - 3x + 3\\
= - 8x\left( {x - 1} \right) - 3\left( {x - 1} \right)\\
= \left( {x - 1} \right)\left( { - 8x - 3} \right)\\
26)8{x^2} - 12x + 2x - 3\\
= 4x\left( {2x - 3} \right) + 2x - 3\\
= \left( {2x - 3} \right)\left( {4x + 1} \right)\\
27)3{x^2} - 2x + 9x - 6\\
= x\left( {3x - 2} \right) + 3\left( {3x - 2} \right)\\
= \left( {3x - 2} \right)\left( {x + 3} \right)\\
28)8{x^2} - 2x + 12x - 3\\
= 2x\left( {4x - 1} \right) + 3\left( {4x - 1} \right)\\
= \left( {4x - 1} \right)\left( {2x + 3} \right)
\end{array}\)
