Question

1)Resolva o sistema
$\left \{ {{x+y=20} \atop {x-y=-10}} \right.$

Answer 1

Isolando x=20-y

Substituindo 20-y-y=-10

                      -2y=-30

                          y=15

 se x=20-y

       x=20-15

       x=5                                                   S={(5;15)}