Cho hai biểu thức  

- Hướng dẫn giải

$\begin{array}{l}A=\left(1-\frac{1}{2}\right).\left(1-\frac{1}{3}\right).\left(1-\frac{1}{4}\right)\mathrm{...}\left(1-\frac{1}{19}\right).\left(1-\frac{1}{20}\right)\\ A=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}\mathrm{....}\frac{18}{19}.\frac{19}{20}\\ A=\frac{1}{20}>\frac{1}{21}\end{array}$

Vậy A>$\frac{1}{21}$