Tìm x, y biết (frac{4}{5}.x = frac{4}{7})

* Hướng dẫn giải

\(\begin{array}{l}
a)\,\frac{4}{5} \cdot x = \frac{4}{7}\\
x = \frac{4}{7}:\frac{4}{5}\\
x = \frac{4}{7}:\frac{5}{4}\\
x = \frac{{4.5}}{{7.4}}
\end{array}\)

Vậy \({\rm{ x  =  }}\frac{5}{7}\)

b) 

\(\begin{array}{l}
\frac{{ - 4}}{8} = \frac{x}{{ - 10}} = \frac{{ - 7}}{y}\\
 \Rightarrow \frac{{ - 1}}{2} = \frac{x}{{ - 10}} = \frac{{ - 7}}{y}
\end{array}\)

Vì \(\frac{{ - 1}}{2} = \frac{x}{{ - 10}} \Rightarrow x = \frac{{( - 1).( - 10)}}{2} = 5\)

Ta có: 

\(\frac{{ - 1}}{2} = \frac{{ - 7}}{y} \Rightarrow y = \frac{{2.( - 7)}}{{ - 1}} = 14\)