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

Theorem

 * $\ds \int \frac {x^3 \rd x} {x^4 + a^4} = \frac {\map \ln {x^4 + a^4} } 4$

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

So: