a) x²+2020x-2021
⇔ x² - x + 2021x - 2021
⇔ x ( x-1 ) + 2021 ( x-1)
⇔ ( x + 2021 ) ( x-1)
⇔$\left[\begin{matrix} x+2021=0\\ x-1=0$\end{matrix}\right.$
⇔$\left[\begin{matrix} x=-2021\\x=1\end{matrix}\right.$
Vậy .......
b) X²+2x-15
⇔ x² - 3x + 5x -15
⇔x (x-3) + 5 (x-3)
⇔ (x+5) (x-3)
⇔ $\left[\begin{matrix} x+5=0\\ x-3=0\end{matrix}\right.
⇔$\left[\begin{matrix} x= -5\\ x=3\end{matrix}\right.$
Vậy ........
c) X²-2x-15
⇔ x² + 3x - 5x - 15
⇔ x( x+3 ) - 5 ( x+3 )
⇔ ( x-5 ) (x+3)
⇔ x=5 và x=-3