Question 29If $f(x)=\log\left(\dfrac{1+x}{1-x}\right)$ for $-1\lt x\lt1,$ then $f\left(\dfrac{3x+x^3}{1+3x^2}\right)$ in terms of $f(x)$ is:$-f(x)$$2f(x)$$3f(x)$$[f(x)]^2$$[f(x)]^3-f(x)$