Câu hỏi 29Nếu $f(x)=\log\left(\dfrac{1+x}{1-x}\right)$ cho $-1\lt x\lt1,$ thì $f\left(\dfrac{3x+x^3}{1+3x^2}\right)$ xét theo $f(x)$ là:$-f(x)$$2f(x)$$3f(x)$$[f(x)]^2$$[f(x)]^3-f(x)$