Ta có :
$\begin{array}{l}
\dfrac{1}{2} = \dfrac{1}{{1.2}} = 1.\dfrac{1}{2}\,\,;\, \dfrac{1}{6} = \dfrac{1}{{2.3}} = \dfrac{1}{2}.\dfrac{1}{3}\,\,;\\
\dfrac{1}{{12}} = \dfrac{1}{{3.4}} = \dfrac{1}{3}.\dfrac{1}{4}\,\,; \dfrac{1}{{20}} = \dfrac{1}{{4.5}} = \dfrac{1}{5}.\dfrac{1}{5}\,\,;\,\\
\dfrac{1}{{30}} = \dfrac{1}{{5.6}} = \dfrac{1}{5}.\dfrac{1}{6}\,\,;
\end{array}$