Question

Resolva os sistemas: x+y=1 x-y=7

Answer 1

x + y = 1
x - y = 7    => x = 7+y

(7+y) +y = 1
2y = -6
y = -3


x - 3 = 1
x = 4

Answer 2

X+Y=1
X-Y=7
 isolando X: X= 1 - Y

Substituindo:
(1-Y)-Y= 7
1-2Y=7
-2Y=7-1
-2Y= 6 *(-1)
2Y=-6
Y=-6/2
Y=-3

X + (-3)=1
X-3= 1
X= 3+1
X=4