Track / Stage 4 / 150 of 340 #410 of 1964

Problem 410

AMC 12 late, AIME early

Algebra Difficulty 4.7 Find the answer

If $f(x)=\lg \frac{1+x}{1-x} \quad(-1<x<1)$ , then $f\left(\frac{3 x+x^{2}}{1+3 x^{2}}\right)$ expressed in terms of $f(x)$ is
(A) $-f(x)$
(B) $2 f(x)$
(C) $3 f(x)$
(D) $[f(x)]^{2}$

The source for this one didn't record the answer, so there is nothing to check what you type against. Work it on paper and mark yourself against the solution below.

Official solution

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.