Đáp án:
$\begin{array}{l}
a)25{x^2} - 10xy + {y^2}\\
= {\left( {5x} \right)^2} - 2.5x.y + {y^2}\\
= {\left( {5x - y} \right)^2}\\
b)2{x^2}{y^2} - 6\sqrt 2 xy + 9\\
= {\left( {\sqrt 2 xy - 3} \right)^2}\\
c)4{y^2} + 4y + 1 = {\left( {2y + 1} \right)^2}\\
d)8{x^3} + 36{x^2}y + 54x{y^2} + 27{y^3}\\
= {\left( {2x + 3y} \right)^3}\\
e)27{y^3} - 27{y^2}x + 9x{y^2} - {x^3}\\
= {\left( {3y - x} \right)^3}\\
g){\left( {xy + 4} \right)^2} - {\left( {2x + 2y} \right)^2}\\
= \left( {xy + 4 - 2x - 2y} \right)\left( {xy + 4 + 2x + 2y} \right)\\
= \left( {x - 2} \right)\left( {y - 2} \right)\left( {x + 2} \right)\left( {y + 2} \right)
\end{array}$