xy+2(x-y)=10;
5xy-3(x-y)=11;
xy=u; x-y=v;
u+2v=10;
5u-3v=11;
u=10-2v;
50-10v-3v=11;
13v=39;
u=4;
v=3.
xy=4;
x-y=3;
x=4/y;
4-y²=3y;
y²+3y-4=0;
x₁=-1; x₂=4;
y₁=-4; y₂=1.
xy+2(x-y)=10;
5xy-3(x-y)=11;
xy=u; x-y=v;
u+2v=10;
5u-3v=11;
u=10-2v;
50-10v-3v=11;
u=10-2v;
13v=39;
u=4;
v=3.
xy=4;
x-y=3;
x=4/y;
4-y²=3y;
x=4/y;
y²+3y-4=0;
x₁=-1; x₂=4;
y₁=-4; y₂=1.