Values which makes the outcome Rational

[Atran]

Hi,

4d = x4 + 6x2y2 + y4 = (x2 + y2)2 + (2xy)2

I want 2(d1/2) to be a rational value, how to assign (x, y) which results a rational 2(d1/2)?
I can't factorize the long expression to one-term expression.

Thanks...