Đáp án:
$\begin{array}{l}
a){\left( {3b + \dfrac{{5a}}{6}} \right)^2}\\
= 9{b^2} + 2.3b.\dfrac{{5a}}{6} + \dfrac{{25{a^2}}}{{36}}\\
= 9{b^2} + 5ab + \dfrac{{25{a^2}}}{{36}}\\
b){\left( {5x - y} \right)^2} = 25{x^2} - 10xy + {y^2}\\
c)\left( {2a + b - 5} \right)\left( {2a - b + 5} \right)\\
= {\left( {2a} \right)^2} - {\left( {b - 5} \right)^2}\\
= 4{a^2} - \left( {{b^2} - 10b + 25} \right)\\
= 4{a^2} - {b^2} + 10b - 25\\
d)\left( {{x^2} + \dfrac{2}{5}y} \right)\left( {{x^2} - \dfrac{2}{5}y} \right)\\
= {\left( {{x^2}} \right)^2} - {\left( {\dfrac{2}{5}y} \right)^2}\\
= {x^4} - \dfrac{{4{y^2}}}{{25}}
\end{array}$
