`( 1 ) <=> x^2 - xy + y^2 = 3`

`( 2 ) <=> ( x + y )( x^2 - xy + y^2 ) - x^2 + 2xy - 2x - 5y = 0`

`3x + 3y - x^2 + 2xy + 2xy - 2x - 5y = 0`

`x^2 - x - 2xy + 2y = 0`

`x( x - 1 ) - 2y( x - 1 ) = 0`

`( x - 2y )( x - 1 ) = 0`

`x = 1 ( 3 )  hoặc   x = 2y  ( 4 )`

`( 1 ) ; ( 3 ) => y^2 - y + 1 = 3`

`y^2 - y - 2 = 0`

`( y - 2 )( y + 1 ) = 0`

`y = 2  hoặc  y = -1`

`( 1 ) ; ( 4 ) => 4y^2 + y^2 - 2y^2 = 3`

`3y^2 = 3`

`y^2 = 1`

`y = +- 1 => x = +- 2`

`Vậy  ( x ; y ) in { ( 2 ; 1 ) ; ( -2 ; -1 ) ; ( 1 ; -1 ) ; ( 1 ; 2 ) }`

`=>` có `4` nghiệm