Đáp án: 21
Giải thích các bước giải:
$\begin{array}{l}
\left( {2 + 5x} \right){\left( {1 - \frac{x}{2}} \right)^8}\\
= \sum\limits_{k = 0}^8 {.C_8^k.{{\left( { - \frac{1}{2}} \right)}^k}.{x^k}.\left( {2 + 5x} \right)} \\
= \sum\limits_{k = 0}^8 {2.C_8^k.{{\left( { - \frac{1}{2}} \right)}^k}.{x^k} + \sum\limits_{k = 0}^8 {5.C_8^k.{{\left( { - \frac{1}{2}} \right)}^k}.{x^{k + 1}}} } \\
Hệ\,số:{x^3}:2C_8^3.{\left( { - \frac{1}{2}} \right)^3} + 5.C_8^2.{\left( { - \frac{1}{2}} \right)^2} = 21
\end{array}$
