Quy đồng mẫu những phân số sau: − 3/20 ; − 2/15 ; − 7/− 30 .

BCNN\[\left( {20,\,\,15,\,\,30} \right) = 60\].

Do đó:

\(\frac{{ - 3}}{{20}} = \frac{{ - 3.3}}{{20.3}} = \frac{{ - 9}}{{60}}\);

\(\frac{{ - 2}}{{15}} = \frac{{ - 2.4}}{{15.4}} = \frac{{ - 8}}{{60}}\);

\(\frac{{ - 7}}{{ - 30}} = \frac{7}{{30}} = \frac{{7.2}}{{30.2}} = \frac{{14}}{{60}}\).