Đáp án:
Đặt A = x + y
B = x - y
=> $\left \{ {{4A=5B} \atop {40/A + 40/B = 9}} \right.$
<=> $\left \{ {{A=5/4.B} \atop {40/(5/4.B) + 40/B = 9}} \right.$
<=> $\left \{ {{A=5/4.B} \atop {32/B + 40/B = 9}} \right.$
<=> $\left \{ {{A=5/4.B} \atop {72/B = 9}} \right.$
<=> $\left \{ {{A=5/4.B} \atop {B=8}} \right.$
<=> $\left \{ {{A=10} \atop {B=8}} \right.$
<=> $\left \{ {{x+y=10} \atop {x-y=8}} \right.$
<=> $\left \{ {{x=9} \atop {y=1}} \right.$
Vậy x=9 và y=1
