Bài 38 trang 23 SGK Toán 6 tập 2

a) Thời gian nào dài hơn: \(\dfrac{2}{3}h\) hay \(\dfrac{3}{4}h\) ?

b) Đoạn thẳng nào ngắn hơn: \(\dfrac{7}{{10}}m\) hay \(\dfrac{3}{{4}}m\) ?

c) Khối lượng nào lớn hơn: \(\dfrac{7}{8}kg\) hay \(\dfrac{9}{10}kg\)

d) Vận tốc nào nhỏ hơn : \(\dfrac{5}{6}km/h\) hay \(\dfrac{7}{9}km/h\) 

a)

Ta có:

\(\begin{array}{l} \dfrac{2}{3} = \dfrac{{2.4}}{{3.4}} = \dfrac{8}{{12}};\\ \dfrac{3}{4} = \dfrac{{3.3}}{{4.3}} = \dfrac{9}{{12}}; \end{array}\)

Mà:  \(\dfrac{8}{{12}} < \dfrac{9}{{12}}\,\,\left( {do\,\,\,8 < 9} \right)\)

Nên: \(\dfrac{2}{3}\)h < \(\dfrac{3}{4}\)h ;                    

b) Ta có:

\(\begin{array}{l}
\dfrac{7}{{10}} = \dfrac{{7.2}}{{10.2}} = \dfrac{{14}}{{20}};\\
\dfrac{3}{4} = \dfrac{{3.5}}{{4.5}} = \dfrac{{15}}{{20}};\\
\dfrac{{14}}{{20}} < \dfrac{{15}}{{20}}\,\,\left( {do\,\,\,14 < 15} \right)\\
 \Rightarrow \dfrac{7}{{10}} < \dfrac{3}{4}
\end{array}\)

Vậy: \(\dfrac{7}{10}\)m < \(\dfrac{3}{4}\)m ;                

c)  Ta có: 

\(\begin{array}{l}
\dfrac{7}{8} = \dfrac{{7.5}}{{8.5}} = \dfrac{{35}}{{40}};\\
\dfrac{9}{{10}} = \dfrac{{9.4}}{{10.4}} = \dfrac{{36}}{{40}};\\
\dfrac{{35}}{{40}} < \dfrac{{36}}{{40}}\,\,\left( {do\,\,\,35 < 36} \right)\\
 \Rightarrow \dfrac{7}{8} < \dfrac{9}{{10}}
\end{array}\)

Vậy\(\dfrac{7}{8}\)kg < \(\dfrac{9}{10}\)kg ;              

d) Ta có:

\(\begin{array}{l}
\dfrac{5}{6} = \dfrac{{5.9}}{{6.9}} = \dfrac{{45}}{{54}};\\
\dfrac{7}{9} = \dfrac{{7.6}}{{9.6}} = \dfrac{{42}}{{54}};\\
\dfrac{{45}}{{54}} > \dfrac{{42}}{{54}}\,\,\left( {do\,\,\,45 > 42} \right)\\
 \Rightarrow \dfrac{5}{6} > \dfrac{7}{9}
\end{array}\)

Vậy \(\dfrac{5}{6}\) km/h > \(\dfrac{7}{9}\) km/h.