Question

Solve x+y=3; x-y=1.
X+y=0;x-y=2.
2x+y=3;2x-y=1.

Answer 1

1.X-Y=1 or, x=1+y; putting the value of x in other equation,
 x+y=3 or, 1+y+y=3 or,2y=2 or, y=1,so x=1+1=2.
2. x+y=0 or, x= -y; so putting the value of x,
 x-y=2 or, -y-y=2 or, -2y=2 or, y=-1, so x=-(-1)=1.
3. 2x-y=1 or, x=(1+y)/2; so putting the value of x,
2x+y=3 or, 2(1+y)/2+y=3 or, 2y=2 or, y=1, so x=(1+1)/2=1.

Answer 2

1   x+y=3............................1
      x=1+y...........................2
so 2y=2             so  y=1
                               x=2
2   x+y=0........................1
      x=2+y......................2
 so   2y=-2         so  y=-1
                              x=1
3  2x+y=3.........................1
    y=2x-1..............................2
s0    4x=4           so x=1
                               y=1