Question

Calcule os sistema
{×+y=20
{×_y=30

Answer 1

x + y = 20
x - y = 30
--------------
2x     = 50
x = 50/2
x = 25

x + y = 20
25 + y = 20
y = 20 -25
y = -5

Resposta: 
x = 25
y = -5

Answer 2

x+y=20
x-y=30
¨¨¨¨¨¨¨¨¨¨
2x=50
x=50/2
x=25

x+y=20
25+y=20
y+20-25
y=-5