Primitive of x cubed over x fourth plus a fourth/Proof 2

Proof
From Primitive of $\dfrac {x^{n - 1} } {x^n + a^n}$:
 * $\ds \int \frac {x^{n - 1} \rd x} {x^n + a^n} = \frac 1 n \ln \size {x^n + a^n} + C$

So: