Đáp án:
Chọn B.
Giải thích các bước giải:
\[\begin{array}{l}
x{\left( {3x - 1} \right)^6} + {\left( {2x - 1} \right)^8}\\
= x\sum\limits_{k = 0}^6 {C_6^k{{\left( {3x} \right)}^k}{{\left( { - 1} \right)}^{6 - k}}} + \sum\limits_{i = 0}^8 {C_8^i{{\left( {2x} \right)}^i}{{\left( { - 1} \right)}^{8 - i}}} \\
= x\sum\limits_{k = 0}^6 {C_6^k{3^k}{{\left( { - 1} \right)}^{6 - k}}{x^k} + \sum\limits_{i = 0}^8 {C_8^i{2^i}{{\left( { - 1} \right)}^{8 - i}}{x^i}} } \\
De\,\,co\,\,he\,\,\,so\,\,\,cua\,\,\,{x^5} \Rightarrow \left\{ \begin{array}{l}
k = 4\\
i = 5
\end{array} \right.\\
\Rightarrow he\,\,so\,\,\,cua\,\,\,{x^5}\,\,\,la:\,\,\,C_6^4{3^4}.{\left( { - 1} \right)^2} + C_8^5{2^5}.{\left( { - 1} \right)^3} = - 577.
\end{array}\]
