a)
[tex]\left \{ {{x+y=8} \atop {x-y=2}} \right. \\ - - - - - - - - -\boxed{+} \\ x + x = 8 + 2 \\ 2x = 10 \\ x = \dfrac{10}{2} \\ \boxed{ x = \bf 5} \\ \\ x + y = 8 \\ 5 + y = 8 \\ y = 8 - 5 \\ \boxed{ y = \bf3} \\ \\ \large\green{ \bf \boxed{S = \{(5,3) \}}}[/tex]
b)
[tex]\left \{ {{x+y=20} \atop {x-y=8}} \right. \\ - - - - - - - - - \boxed{+} \\ 2x = 28 \\ x = \dfrac{28}{2} \\ \boxed{ x \bf \: = 14} \\ \\ x + y = 20 \\ 14 + y = 20 \\ y = 20 - 14 \\ \boxed{ y = \bf 6} \\ \\ \boxed{ \bf \large \green{ S = \{(14,6) \}}} \\ \\ \boxed{ \large \blue{ \bf \: \underline{\bf Bons \: Estudos!}}}[/tex]
