Giải thích các bước giải:
$\begin{array}{l}
b)\left( {x - 5} \right)\left( {2x + 3} \right) - 2x\left( {x - 3} \right) + x + 7\\
= 2{x^2} - 7x - 15 - 2{x^2} + 6x + x + 7\\
= - 8\\
c){\left( {a + b} \right)^2} - {\left( {a - b} \right)^2}\\
= \left( {a + b - \left( {a - b} \right)} \right)\left( {a + b + a - b} \right)\\
= 2b.2a\\
= 4ab\\
d){\left( {2x + 1} \right)^2} + {\left( {2x - 1} \right)^2} - 2\left( {1 + 2x} \right)\left( {2x - 1} \right)\\
= {\left( {2x + 1 - \left( {2x - 1} \right)} \right)^2}\\
= {2^2}\\
= 4\\
e){\left( {x - 1} \right)^3} - \left( {x + 2} \right)\left( {{x^2} - 2x + 4} \right) + 3\left( {x - 1} \right)\left( {x + 1} \right)\\
= {\left( {x - 1} \right)^3} - {\left( {x + 2} \right)^3} + 3\left( {{x^2} - 1} \right)\\
= \left( {x - 1 - \left( {x + 2} \right)} \right)\left( {{{\left( {x - 1} \right)}^2} + \left( {x - 1} \right)\left( {x + 2} \right) + {{\left( {x + 2} \right)}^2}} \right) + 3\left( {{x^2} - 1} \right)\\
= - 3\left( {3{x^2} + 3x + 3} \right) - 3\left( {{x^2} - 1} \right)\\
= - 12{x^2} - 9x - 6
\end{array}$
