Cậu tham khảo nhé. Mình làm rất chi tiết mong bạn sẽ hiểu.
$\left \{ {{5x + 6y = 2} \atop {7x + 9y = -1}} \right.$
<=> $\left \{ {{5x = 2 - 6y} \atop {7x + 9y = -1}} \right.$
<=> $\left \{ {{x = \frac{2-6y}{5}} \atop {7.\frac{2-6y}{5} + 9y = -1}} \right.$
<=> $\left \{ {{x=\frac{2-6y}{5}} \atop {\frac{7.(2-6y)}{5} + \frac{9y.5}{5}=\frac{(-1).5}{5} }} \right.$
<=> $\left \{ {{x=\frac{2-6y}{5}} \atop {7(2-6y) + 45y=-5}} \right.$
<=> $\left \{ {{x=\frac{2-6y}{5}} \atop {14 - 42y + 45y = -5}} \right.$
<=> $\left \{ {{x=\frac{2-6y}{5}} \atop {3y = -19}} \right.$
<=> $\left \{ {{x=\frac{2-6y}{5}} \atop {y=-\frac{19}{3}}} \right.$
<=> $\left \{ {{x=\frac{2-6.-\frac{19}{3}}{5}} \atop {y=-\frac{19}{3}}} \right.$
<=> $\left \{ {{x=\frac{40}{5}} \atop {y=-\frac{19}{3}}} \right.$
<=> $\left \{ {{x = 8} \atop {y=-\frac{19}{3}}} \right.$
Vậy $\left \{ {{x = 8} \atop {y=-\frac{19}{3}}} \right.$
