\(\displaystyle a)\,{{ - 7} \over {20}} = {7 \over {10}}.{{ - 1} \over 2}\)
Vì \(\dfrac{7}{{10}}.\dfrac{{ - 1}}{2} = \dfrac{{7.\left( { - 1} \right)}}{{10.2}} = \dfrac{{ - 7}}{{20}}\)
\(\displaystyle b)\,{{ - 7} \over {20}} = {7 \over {10}}:(-2)\)
Vì \(\dfrac{7}{{10}}:( - 2) = \dfrac{7}{{10}}:\dfrac{{ - 2}}{1} = \dfrac{7}{{10}}.\dfrac{{ - 1}}{2}\)\(\, = \dfrac{{ - 7}}{{20}}\)
\(\displaystyle c)\,{{ - 7} \over {20}} = {{ - 2} \over 5} + {1 \over {20}}\)
Vì \(\dfrac{{ - 2}}{5} + \dfrac{1}{{20}} = \dfrac{{ - 8}}{{20}} + \dfrac{1}{{20}} = \dfrac{{\left( { - 8} \right) + 1}}{{20}} \)\(\,= \dfrac{{ - 7}}{{20}}\)
\(\displaystyle {\rm{d}})\,{{ - 7} \over {20}} = {{ - 1} \over 5} + {{ - 3} \over {20}}\)
Vì \(\dfrac{{ - 1}}{5} + \dfrac{{ - 3}}{{20}} = \dfrac{{ - 4}}{{20}} + \dfrac{{ - 3}}{{20}} \)\(\,= \dfrac{{\left( { - 4} \right) + \left( { - 3} \right)}}{{20}} = \dfrac{{ - 7}}{{20}}\)
Chú ý: a, b, c có thể có nhiều đáp án.
