Ta có: \(xy=140\Rightarrow x=\dfrac{140}{y}\)
\(5x-y=15\)
\(\Leftrightarrow5.\dfrac{140}{y}-y=15\)
\(\Leftrightarrow\dfrac{700}{y}-\dfrac{y^2}{y}=15\)
\(\Leftrightarrow\dfrac{700-y^2}{y}=15\)
\(\Rightarrow y=20\)
\(\Rightarrow x=\dfrac{140}{y}=\dfrac{140}{20}=7\)
ta có: 5x - y =15 => y = 5x - 15
=> x.(5x - 15) = 140
=> 5x^2 - 15x -140 = 0
=> 5.(x^2 -3x -28) = 0
=> x^2 - 4x + 7x -28 = 0
=> x(x-4) +7.(x-4) = 0
=> (x+7).(x-4)=0
=> x= -7 hoặc x=4
Nếu x=-7 => y= -50
Nếu x=4 => y=5
vậy hệ phương trình có cặp nghiệm là: (x;y)=(-7;-50) hoặc( x;y) = ( 4;5)
