tìm x,y biết x/3=y/4 và 2x+5y=10

Đáp án:

$\begin{array}{l}
\frac{x}{3} = \frac{y}{4}\\
 \Rightarrow x = \frac{{3y}}{4}\,thay\,vao:2x + 5y = 10\\
 \Rightarrow 2.\frac{{3y}}{4} + 5y = 10\\
 \Rightarrow \left( {\frac{3}{2} + 5} \right).y = 10\\
 \Rightarrow \frac{{13}}{2}.y = 10\\
 \Rightarrow y = 10:\frac{{13}}{2}\\
 \Rightarrow y = \frac{{20}}{{13}}\\
 \Rightarrow x = \frac{{3y}}{4} = \frac{3}{4}.\frac{{20}}{{13}} = \frac{{15}}{{13}}\\
\left( {x;y} \right) = \left( {\frac{{15}}{{13}};\frac{{20}}{{13}}} \right)
\end{array}$