So sánh các số hữu tỉ sau:
\(\begin{array}{l}a) – \dfrac{{57}}{{2021}}\,;\dfrac{1}{{6345}}\\b)\dfrac{{ – 19}}{{35}};\dfrac{{ – 13}}{{21}}\\c)\dfrac{6}{{73}};\dfrac{9}{{82}}\end{array}\)
a) So sánh qua số trung gian: 0
b) Quy đồng mẫu số chung: 105
c) Đưa về dạng 2 phân số cùng tử số: 18
a)
\(\begin{array}{l} – \dfrac{{57}}{{2021}} < 0;\dfrac{1}{{6345}} > 0\\ \Rightarrow – \dfrac{{57}}{{2021}} < \dfrac{1}{{6345}}\end{array}\)
b)
\( – \dfrac{{19}}{{35}} =- \dfrac{{19.3}}{{35.3}}= – \dfrac{{57}}{{105}}; – \dfrac{{13}}{{21}} =- \dfrac{{13.5}}{{21.5}} = -\dfrac{{65}}{{105}}\)
Mà -57 > -65 nên \( – \dfrac{{57}}{{105}} > -\dfrac{{ 65}}{{105}}\)
Nên \( – \dfrac{{19}}{{35}} > – \dfrac{{13}}{{21}}\)
c)
\(\dfrac{6}{{73}} =\dfrac{6.3}{{73.3}} = \dfrac{{18}}{{219}};\dfrac{9}{{82}} = \dfrac{9.2}{{82.2}} = \dfrac{{18}}{{164}}\)
Mà 219 > 164 nên \(\dfrac{{18}}{{219}} < \dfrac{{18}}{{164}}\)
Nên \(\dfrac{6}{{73}} < \dfrac{9}{{82}}\)