a) Để A có nghĩa thì :
\(3x^3-x^2-3x+1\ne0\)
\(\Leftrightarrow x^2\left(3x-1\right)-\left(3x-1\right)\ne0\)
\(\Leftrightarrow\left(3x-1\right)\left(x^2-1\right)\ne0\)
\(\Leftrightarrow\left(3x-1\right)\left(x-1\right)\left(x+1\right)\ne0\)
\(\Leftrightarrow\hept{\begin{cases}x\ne\frac{1}{3}\\x\ne1\\x\ne-1\end{cases}}\)
ĐKXĐ : \(\hept{\begin{cases}x\ne\frac{1}{3}&x\ne\pm1&\end{cases}}\)