Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
A = \dfrac{{f\left( {{x_2}} \right) - f\left( {{x_1}} \right)}}{{{x_2} - {x_1}}}\\
= \dfrac{{ - 2{x_2}^2 + 4{x_2} + 1 - \left( { - 2{x_1}^2 + 4{x_1} + 1} \right)}}{{{x_2} - {x_1}}}\\
= \dfrac{{ - 2{x_2}^2 + 4{x_2} + 2{x_1}^2 - 4{x_1}}}{{{x_2} - {x_1}}}\\
= \dfrac{{ - 2.\left( {{x_2}^2 - {x_1}^2} \right) + 4.\left( {{x_2} - {x_1}} \right)}}{{{x_2} - {x_1}}}\\
= \dfrac{{ - 2.\left( {{x_2} - {x_1}} \right)\left( {{x_2} + {x_1}} \right) + 4.\left( {{x_2} - {x_1}} \right)}}{{{x_2} - {x_1}}}\\
= - 2.\left( {{x_2} + {x_1}} \right) + 4
\end{array}\)
