Ta có :
\(\dfrac{x}{y}=\dfrac{3}{4}\Rightarrow\dfrac{x}{3}=\dfrac{y}{4}\)
Cho k với điều kiện :
\(\dfrac{x}{3}=\dfrac{y}{4}=k\Rightarrow x=3k;y=4k\)
⇒ x . y = 3k . 4k = 12k2
⇒ 12k2 = 1200 ⇒ k = 10
⇒x = 10 . 3 = 30
y = 10 . 4 = 40
Vậy ....
Theo đề bài ta có: \(\dfrac{x}{y}=\dfrac{3}{4}\&xy=1200\Rightarrow\dfrac{x}{3}=\dfrac{y}{4}\&xy=1200\)
Đặt \(\dfrac{x}{3}=\dfrac{y}{4}=k\Rightarrow x=3k;y=4k\)
\(\Rightarrow xy=3k.4k=12.k^2\)
\(\Rightarrow12.k^2=1200\Rightarrow k=10\)
\(\Rightarrow\left\{{}\begin{matrix}x=30\\y=40\end{matrix}\right.\)