Question
Integrate the function $e^{3 \log x}\left(x^{4}+1\right)^{-1}$

Answer

Let $\mathrm{I}=\mathrm{e}^{3 \log \mathrm{x}}\left(\mathrm{x}^{4}+1\right)^{-1}$ 
$=e^{\log x^{3}}\left(x^{4}+1\right)^{-1}=\frac{x^{3}}{x^{4}+1}$ 
Let x4 = t $\Rightarrow$ 4x3 dx = dt $\Rightarrow$ x3 dx = $\frac{dt}{4}$
$\Rightarrow \int e^{3 \log x}\left(x^{4}+1\right)^{-1}=\int \frac{x^{3}}{x^{4}+1} d x$ 
= $\int \frac{1}{t+1} \cdot \frac{d t}{4}$ 
= $\frac{1}{4} \cdot \int \frac{1}{t+1} \cdot d t$ 
= $\frac{1}{4} \log (t+1)+C$ 
$\Rightarrow I=\frac{1}{4} \log \left(x^{4}+1\right)+c$

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