x 2+y 2 2 4 x 2-y 2, x 2+y 2 4 x 2-y 2, x2+y2 2 x2-y2, x2+y2 2 a2 x2-y2, x 2+y 2 4, x2-y2 2, x2-y2 6 xy 4, x2+y2+xy 4, x2+y2 4x, x2+y2/4 1, x2+xy+y2 4, x2+y2 x2-y2, x2+y2 3 y4, x2-4x+y2 0, x2/4+y2/3 1, x-2 2+y2 3
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