Tính tích : 2^2/1x3 x 3^2/2x4 x 4^2/3x5 ...... 20^2/19x21

Đáp án:

$\begin{array}{l}
\frac{{{2^2}}}{{1.3}}.\frac{{{3^2}}}{{2.4}}.\frac{{{4^2}}}{{3.5}}...\frac{{{{20}^2}}}{{19.21}}\\
 = \frac{{{2^2}{{.3}^2}{{.4}^2}{{...19}^2}{{.20}^2}}}{{{{1.2.3}^2}{{.4}^2}{{...19}^2}{{.20}^2}.21}}\\
 = \frac{2}{{1.21}}\\
 = \frac{2}{{21}}
\end{array}$