Ответ: [tex] \left \{ {{4x+y=8,} \atop {xy=4}} ight. \ \left \{ {{y=8-4x,} \atop {x(8-4x)=4}} ight. \ \left \{ {{y=8-4x,} \atop {8x-4x^2-4=0;}} ight. \ \left \{ {{y=8-4x,} \atop {x^2-2x+1=0;}} ight. \ \left \{ {{y=8-4x,} \atop {(x-1)^2=0;}} ight. \\ \left \{ {{y=8-4x,} \atop {x-1=0;}} ight. \ \left \{ {{y=8-4x,} \atop {x=1;}} ight. \ \left \{ {{x=1,} \atop {y=4;}} ight. \\ x-2y=1-2\cdot4=1-8=-7.[/tex]