(Tính :)

$\left( 1-\frac{1}{2} \right)\times \left( 1-\frac{1}{3} \right)\times \left( 1-\frac{1}{4} \right)\times ...\times \left( 1-\frac{1}{18} \right)\times \left( 1-\frac{1}{19} \right)\times \left( 1-\frac{1}{20} \right)$$\begin{align}& =\frac{1}{2}\times \frac{2}{3}\times \frac{3}{4}\times ...\times \frac{17}{18}\times \frac{18}{19}\times \frac{19}{20} \\& =\frac{1}{20} \\\end{align}$