+) Vì ${x : y = 3 : 7 }$ => $\frac{x}{3}$ = $\frac{y}{7}$
+) Đặt $\frac{x}{3}$ = $\frac{y}{7}$ = ${k}$ => \(\left[ \begin{array}{l}x=3k\\y=7k\end{array} \right.\)
=> ${3k . 7k = 336}$
=> ${k².(3.7)=336}$
=> ${k² . 21 = 336 }$
=> ${k²= 336 : 21 }$
=> ${k² = 16}$
=> ${k² = 4^{2} }$
=> \(\left[ \begin{array}{l}k=4\\k=-4\end{array} \right.\)
Nếu k = 4
=> \(\left[ \begin{array}{l}x=4.3=12\\y=4.7=28\end{array} \right.\)
Nếu k = -4
=> \(\left[ \begin{array}{l}x=-4.3=12\\y=-4.7=-28\end{array} \right.\)
Vậy \(\left[ \begin{array}{l}x=12\\y=28\end{array} \right.\) ; \(\left[ \begin{array}{l}x=-12\\x=-28\end{array} \right.\)
$\text{HỌC TỐT!}$
