Question

Como resolver o sistema
X+y= 20
X-y= -10

Answer 1

{ x + y = 20
{ x - y = - 10, fazendo por adição (adicionamos as expressões membro a membro)
2x = 10
x = 10 / 2
x = 5

x + y = 20
5 + y = 20
y = 20 - 5
y = 15

Answer 2

$\left \{ {{x+y=20} \atop {x-y=-10}} \right. \\\\\\ x+y=20\to x=-y+20\\\\\\ x-y=-10\to -y+20-y=-10\to -y-y=-10-20\to \\\\ -2y=-30\to y= \frac{-30}{-2} \to y=15\\\\\\ x=-y+20\to x=-15+20\to x=5\\\\\\\\ S= \left \{5;15 \left \}$