Question

X + y = 50} x - y = 0}

Answer 1

$\left \{ {{x+y=50} \atop {x-y=0}} \right. \\\\ x-y=0\to x=y\\\\\\ x+y=50\to y+y=50 \to 2y=50 \to y= \frac{50}{2} \to y=25\\\\\\ x=y\to x=25\\\\\\\\ S= \left \{ 25;25 \left \}$

Answer 2

$\left \{ {{x+y=50} \atop {x-y=50}} \right. \\ \\ \left \{ {{x=50-y} \atop {x-y=50}} \right.$

Substituindo na 2ª equação, temos

$50-y-y=0 \\ \\ -2y=-50 \\ \\ y = \frac{-50}{-2} = 25$

Substituindo na 1ª equação, temos

$x+25=50 \\ \\ x= 50-25 \\ \\ x= 25$