Question

resolva o sistema
{x+y=9
{x-y=29

Answer 1

x + y = 9     (1)
x - y = 29    (2)

(1) + (2)
2x = 38
  x = 38/2
                  x = 19
x em (1) [ se preferir, pode usar (2)]
19 + y = 9
      y = 9 - 19
                         y = - 10

Answer 2

$\left \{ {{x+y=9} \atop {x-y=29}} \right. \\\\\\ x+y=9\to x=-y+9\\\\\\ x-y=29\to -y+9-y=29\to -y-y=29-9\to -2y=20\to \\\\y= \frac{20}{-2} \to y=-10\\\\\\ x=-y+9\to x=-(-10)+9\to x=10+9\to x=19\\\\\\\\ S=\{19;-10\}$