8.
[tex]\it a)\ \ \dfrac{x}{6}=\dfrac{y}{4} \Rightarrow x=\dfrac{6y}{4} \Rightarrow x=1,5y\ \ \ \ \ (*)\ \ \ (rela\c{\it t}ie\ de\ substitu\c{\it t}ie)\\ \\ \\ 2x-5y=8\ \stackrel{(*)}{\Longrightarrow}\ 2\cdot1,5y-5y=8 \Rightarrow -2y=8|_{:(-2)} \Rightarrow y=-4\\ \\ (*) \Rightarrow x=1,5\cdot(-4) \Rightarrow x=-6\\ \\ S=\{(-6;\ -4)\}[/tex]
[tex]\it b)\ \ \dfrac{x}{4}=\dfrac{y}{5} \Rightarrow x=\dfrac{4y}{5} \Rightarrow x=0,8y\ \ \ \ \ (*)\ \ \ (rela\c{\it t}ie\ de\ substitu\c{\it t}ie)\\ \\ \\ 5x-3y=5\ \stackrel{(*)}{\Longrightarrow} 5\cdot0,8y-3y=5 \Rightarrow 4y-3y=5 \Rightarrow y=5\\ \\ (*) \Rightarrow x=0,8\cdot5 \Rightarrow x=4\\ \\ S=\{(4;\ 5)\}[/tex]
