Answer
$$\frac{1}{2}\sin^{-1} x^2+c.$$
Work Step by Step
Let $u=x^2$, then $du=2xdx$. Now, we have
$$\int \frac{dx}{\sqrt{1-x^4}}=\frac{1}{2}\int \frac{du}{\sqrt{1-u^2}}\\
=\frac{1}{2}\sin u+c=\frac{1}{2}\sin^{-1} x^2+c.$$
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