Question

из системы уравнений
0,6(х-у)=12
0,7(х+у)=21 Найти х*у

Answer 1

$\left \{ {{0,6(x-y)=12} \atop {0.7(x+y)=21}} \right. \left \{ {{0.6x-0.6y=12} \atop {0.7x+0.7y=21}} \right. 0.6x=12+0.6y x=20+y 0.7(20+y)+0.7y=21 14+0.7y+0.7y=21 1.4y=21-14 1.4y=7 y=7/1.4 y=5 x=20+5=25 xy=25*5=125$

Answer 2

$\left \{ {{0.6(x-y)=12} \atop {0.7(x+y)=21}} \right.$

$\left \{ {{x-y=20} \atop {x+y=30}} \right.$

решаем методом сложения:

$\left \{ {{x+y=30} \atop {2x=50}} \right.$

$\left \{ {{x=25} \atop {y=30-x}} \right.$

$\left \{ {{x=25} \atop {y=5}} \right.$

$x*y=25*5=125$

Ответ: 125