Question

решите систему уравнений методом подстановки​

Answer 1

$\displaystyle\bf\left \{ {{3x+4y=55} \atop {7x-y=56}} \right. \\\\\\ \left \{ {{3x+4y=55} \atop {y=7x-56}} \right.\\\\\\ \left \{ {{3x+4\cdot(7x-56)=55} \atop {y=7x-56}} \right.\\\\\\ \left \{ {{3x+28x-224=55} \atop {y=7x-56}} \right. \\\\\\ \left \{ {{31x=279} \atop {y=7x-56}} \right. \\\\\\ \left \{ {{x=9} \atop {y=7\cdot 9-56}} \right. \\\\\\ \left \{ {{x=9} \atop {y=7}} \right. \\\\\\ Otvet:(9 \ ; \ 7)$