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

[latex] \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 [/latex]

[latex] \left \{ {{0.6(x-y)=12} \atop {0.7(x+y)=21}} \right. [/latex] [latex] \left \{ {{x-y=20} \atop {x+y=30}} \right. [/latex] решаем методом сложения: [latex] \left \{ {{x+y=30} \atop {2x=50}} \right. [/latex] [latex] \left \{ {{x=25} \atop {y=30-x}} \right. [/latex] [latex] \left \{ {{x=25} \atop {y=5}} \right. [/latex] [latex]x*y=25*5=125[/latex] Ответ: 125