Giải phương trình nghiệm nguyên: 2x^2 + y^2 - 2xy - 2x + y = 4

\(2{x^2} + {y^2} - 2xy - 2x + y = 4 \Leftrightarrow 2\left( {2{x^2} + {y^2} - 2xy - 2x + y} \right) = 2.4\)

\( \Leftrightarrow 4{x^2} + 2{y^2} - 4xy - 4x + 2y = 8 \Leftrightarrow \left( {4{x^2} + {y^2} + 1 - 4xy - 4x + 2y} \right) + {y^2} = 8 + 1\)

\( \Leftrightarrow {\left( {2x - y - 1} \right)^2} + {y^2} = 9\)

Vì x; y ∈ ℤ \( \Rightarrow {\left( {2x - y - 1} \right)^2};{y^2} \in Z \Rightarrow {\left( {2x - y - 1} \right)^2} + {y^2} = 9 = {\left( { - 3} \right)^2} + {0^2} = {3^2} + {0^2}\)

TH1: \(\left\{ {\begin{array}{*{20}{c}}{{{\left( {2x - y - 1} \right)}^2} = {{\left( { - 3} \right)}^2}}\\{{y^2} = {0^2}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{x = - 1\left( {TM} \right)}\\{y = 0\left( {TM} \right)}\end{array}} \right.\)

TH2:\(\left\{ {\begin{array}{*{20}{c}}{{{\left( {2x - y - 1} \right)}^2} = {3^2}}\\{{y^2} = {0^2}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{x = 2\left( {TM} \right)}\\{y = 0\left( {TM} \right)}\end{array}} \right.\)

TH3: \(\left\{ {\begin{array}{*{20}{c}}{{{\left( {2x - y - 1} \right)}^2} = {0^2}}\\{{y^2} = {{\left( { - 3} \right)}^2}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{x = - 1\left( {TM} \right)}\\{y = - 3\left( {TM} \right)}\end{array}} \right.\)

TH4: \(\left\{ {\begin{array}{*{20}{c}}{{{\left( {2x - y - 1} \right)}^2} = {0^2}}\\{{y^2} = {3^2}}\end{array}} \right. \Leftrightarrow \left\{ {\begin{array}{*{20}{c}}{x = 2\left( {TM} \right)}\\{y = 3\left( {TM} \right)}\end{array}} \right.\)

Vậy \(\left( {x;y} \right) = \left\{ {\left( { - 1;0} \right);\left( {2;0} \right);\left( { - 1; - 3} \right);\left( {2;3} \right)} \right\}\).