Đáp án:
$\begin{array}{l}
21)\left( {{x^2} + 1} \right)\left( {{x^4} - {x^2} + 1} \right)\\
= {\left( {{x^2}} \right)^3} + {1^3}\\
= {x^6} + 1\\
22){\left( {\dfrac{1}{5} - x} \right)^2}\\
= \dfrac{1}{{25}} - 2.x.\dfrac{1}{5} + {x^2}\\
= {x^2} - \dfrac{2}{5}x + \dfrac{1}{{25}}\\
23)\left( {2x - 1} \right)\left( {4{x^2} + 2x + 1} \right)\\
= {\left( {2x} \right)^3} - {1^3}\\
= 8{x^3} - 1\\
24)\\
\left( {\dfrac{1}{2}x - \dfrac{1}{3}y} \right)\left( {\dfrac{1}{2}x + \dfrac{1}{3}y} \right)\\
= {\left( {\dfrac{1}{2}x} \right)^2} - {\left( {\dfrac{1}{3}y} \right)^2}\\
= \dfrac{1}{4}{x^2} - \dfrac{1}{9}{y^2}\\
25)\dfrac{1}{{25}} - 4{y^2}\\
= \left( {\dfrac{1}{5} - 2y} \right)\left( {\dfrac{1}{5} + 2y} \right)
\end{array}$
