# A simple substitution for $$x$$. Isn't it?

Geometry Level 3

$\large \begin{cases} f(x) = \ln \left( \dfrac{1+x}{1-x} \right), & \quad f : (-1,1) \to \mathbb{R} \\ g(x) = \dfrac{3x+x^3}{1+3x^2}, & \quad g : \mathbb{R} \to (-1,1) \end{cases}$

For $$f(x)$$ and $$g(x)$$ as defined above, what is $$f \circ g (x)$$ or $$f(g(x))$$?

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