Đáp án:
$\begin{array}{l}
1c)\left( {2a + b - 5} \right)\left( {2a - b + 5} \right)\\
= {\left( {2a} \right)^2} - {\left( {b - 5} \right)^2}\\
= 4{a^2} - {b^2} + 10b - 25\\
d)\left( {{x^2} + \dfrac{2}{5}y} \right)\left( {{x^2} - \dfrac{2}{5}y} \right)\\
= {\left( {{x^2}} \right)^2} - {\left( {\dfrac{2}{5}y} \right)^2}\\
= {x^4} - \dfrac{4}{{25}}{y^2}\\
B2)b)\dfrac{1}{4}{x^2} + 2x{y^2} + 4{y^4}\\
= {\left( {\dfrac{1}{2}x} \right)^2} + 2.\dfrac{1}{2}x.2{y^2} + {\left( {2{y^2}} \right)^2}\\
= {\left( {\dfrac{1}{2}x + 2{y^2}} \right)^2}
\end{array}$
