Primitive of Reciprocal of x fourth plus a fourth/Lemma 2

Lemma for Primitive of Reciprocal of $x^4 + a^4$

 * $\displaystyle \int \frac {\mathrm d x} {x^2 - a x \sqrt 2 + a^2} = \frac {-\sqrt 2} a \arctan \left({1 - \frac {x \sqrt 2} a}\right)$

Proof
The discriminant of $x^2 - a x \sqrt 2 + a^2$ is:

Thus: