Track / Stage 4 / 114 of 340 #374 of 1964

Problem 374

AMC 12 late, AIME early

Algebra Difficulty 4.7 Find the answer

8. 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) $[f(x)]^{2}$

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Official solution

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.