X^2 - 2xy - 4 + y^2 

1, x2 - 2xy - 4 + y2 

2, 4 - x2 - 2xy - y2 

3, x2 + 4y2 - 4xy - 4 

4, x2 - 2xy - 25 + y2

5, 9 - x2 - 2xy - y2

6, x2 + 4y2 - 4xy - 4 

7, x2 - 2xy + y2 - 1 

8, x2 - 2xy + y2 - 4

9, x2 - 2xy + y2 - 49 

10, x2 + 2xy + y2 - 25

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