Tính B=2^2/3.3^2/8.4^2/15.5^2/24.6^2/35.7^2/48.8^2/63.9/80

* Hướng dẫn giải

Đáp án cần chọn là: A

$\begin{array}{l}B=\frac{2^2}{3}\cdot \frac{3^2}{8}\cdot \frac{4^2}{15}\cdot \frac{5^2}{24}\cdot \frac{6^2}{35}\cdot \frac{7^2}{48}\cdot \frac{8^2}{63}\cdot \frac{9^2}{80}\\ =\frac{2.2}{1.3}\cdot \frac{3.3}{2.4}\cdot \frac{4.4}{3.5}\cdot \frac{5.5}{4.6}\cdot \frac{6.6}{5.7}\cdot \frac{7.7}{6.8}\cdot \frac{8.8}{7.9}\cdot \frac{9.9}{8.10}\\ =\frac{\mathrm{2.3.4.5.6.7.8.9}}{\mathrm{1.2.3.4.5.6.7.8.}}\cdot \frac{\mathrm{2.3.4.5.6.7.8.9}}{\mathrm{3.4.5.6.7.8.9.10}}\\ =\frac{9}{1}\cdot \frac{2}{10}=\frac{9.2}{1.10}=\frac{9}{5}\end{array}$