(2x + 1)(5x - 1) = 20x² - 16x - 1
⇔ 10x² + 5x - 2x - 1 = 20x² - 16x - 1
⇔ 10x² + 5x - 2x - 20x² + 16x = 1 - 1
⇔ -10 x² + 19x = 0
⇔ x ( -10 x + 19 ) = 0
⇔ \(\left[ \begin{array}{l}x=0\\-10x + 19 =0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=0\\x=1,9\end{array} \right.\)
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