Question

use  metodo adição para resolver os sistemas: x+y=42    x-y=12

Answer 1

$\left \{ {{x+\not{y}=42} \atop {x-\not{y}=12}} \right.$

$2x=54\\x= \frac{54}{2} \\x=27$

$x+y=42\\y=42-x\\y=42-27\\y=15$