Đáp án:
B=$\frac{2}{3}$
Giải thích các bước giải:
B= \(\begin{array}{l}
\frac{{7!.4!}}{{10!}}.\left( {\frac{{8!}}{{3!.5!}} - \frac{{9!}}{{2!.7!}}} \right)\\
= \frac{{7!.4!}}{{10!}}.\frac{{8!}}{{3!.5!}} - \frac{{7!.4!}}{{10!}}.\frac{{9!}}{{2!.7!}}\\
= \frac{{7.6.5!.4.3!}}{{10.9.8!}}.\frac{{8!}}{{3!.5!}} - \frac{{7!.4.3.2!}}{{10.9!}}.\frac{{9!}}{{2!.7!}}\\
= \frac{{7.6.4}}{{10.9}} - \frac{{4.3}}{{10}} = \frac{{28}}{{15}} - \frac{6}{5} = \frac{2}{3}
\end{array}\)
