Ответ:
(x; y)=(1; 2) х+у=3
Пошаговое объяснение:
\left \{ {{2x+5y=12} \atop {x=-\frac{5}{3}+\frac{4}{3}y}} \right.\\\\2(-\frac{5}{3}+\frac{4}{3}y)+5y=12\\\\-\frac{10}{3}+\frac{8}{3}y+5y=12\\\\-\frac{10}{3}+\frac{23}{3}y=12\\\\ -10+23y=36\\\\ 23y=36+10\\\\23y=46\\\\y=46:23\\\\y=2" alt="\left \{ {{2x+5y=12} \atop {3x-4y=-5}} \right.=>\left \{ {{2x+5y=12} \atop {x=-\frac{5}{3}+\frac{4}{3}y}} \right.\\\\2(-\frac{5}{3}+\frac{4}{3}y)+5y=12\\\\-\frac{10}{3}+\frac{8}{3}y+5y=12\\\\-\frac{10}{3}+\frac{23}{3}y=12\\\\ -10+23y=36\\\\ 23y=36+10\\\\23y=46\\\\y=46:23\\\\y=2" align="absmiddle" class="latex-formula">
Сумма:
1+2=3