Đáp án:
$\begin{array}{l}
a)A = {26^2} - {24^2}\\
= \left( {26 - 24} \right).\left( {26 + 24} \right)\\
= 2.50\\
B = {27^2} - {25^2}\\
= \left( {27 - 25} \right)\left( {27 + 25} \right)\\
= 2.52\\
Do:2.50 < 2.52\\
\Rightarrow A < B\\
b)A = 2021.2023 + 2023.2025\\
= 2023.\left( {2021 + 2025} \right)\\
= 2023.4046\\
= 2023.2.2023\\
= {2.2023^2}\\
B = {2020^2} + {2024^2}\\
\Rightarrow A - B\\
= {2.2023^2} - {2020^2} - {2024^2}\\
= \left( {{{2023}^2} - {{2020}^2}} \right) - \left( {{{2024}^2} - {{2023}^2}} \right)\\
= 4043 - 4047\\
= - 4 < 0\\
\Rightarrow A - B < 0\\
\Rightarrow A < B
\end{array}$
