[tex]\displaystyle \\
x:4 + y:4 = 4 ~~~~~~~\text{unde }x,~y \neq 0\\ \\
\frac{x}{4} + \frac{y}{4} =4 \\ \\
\frac{x+y}{4} =4 \\
x+y = 4 \cdot 4 \\
x+y = 16 \\ \\
x = 16 -y \\ \\
\text{Dam valori lui y de la 1 la 15 si il calculam pe x. } \\
y_1 = 1 ;~~~ x_1 = 15 \\
y_2 = 2 ;~~~ x_2 = 14 \\
y_3 = 3 ;~~~ x_3 = 13 \\
y_4 = 4 ;~~~ x_4 = 12 \\
........................... \\
y_{14} = 14 ;~~~ x_{14} = 2 \\
y_{15} = 15 ;~~~ x_{15} = 1 \\
[/tex]