Giải thích các bước giải:
$\begin{array}{l}
11)\left( {x + y} \right)\left( {{x^2} - xy + {y^2}} \right) = {x^3} + {y^3}\\
12)\left( {x - 5} \right)\left( {{x^2} - 6x + 1} \right) = {x^3} - 11{x^2} + 31x - 5\\
13)\left( {2{x^2} - 1} \right)\left( {3{x^2} - x + 2} \right) = 6{x^4} - 5{x^3} + {x^2} + x - 2\\
14)\left( {2 - 3{x^2}} \right)\left( {{x^3} + 2{x^2} - 3} \right) = - 3{x^5} - 6{x^4} + 2{x^3} + 13{x^2} - 6\\
15)\left( {\frac{1}{2}x + y} \right)\left( {\frac{1}{2}x - y} \right) = {\left( {\frac{1}{2}x} \right)^2} - {y^2} = \frac{{{x^2}}}{4} - {y^2}\\
17)\left( {x - \frac{1}{2}y} \right)\left( {x + \frac{1}{2}y} \right) = {x^2} - {\left( {\frac{1}{2}y} \right)^2} = {x^2} - \frac{1}{4}{y^2}\\
18)\left( {9x - 2} \right)\left( {{x^2} - 3x + 5} \right) = 9{x^3} - 29{x^2} + 54x - 10\\
19)\left( {7x - 1} \right)\left( {2{x^2} - 5x + 3} \right) = 14{x^3} - 37{x^2} + 26x - 3\\
20)\left( {5x + 3} \right)\left( {3{x^2} + 6x + 7} \right) = 15{x^3} + 39{x^2} + 53x + 21
\end{array}$
