a/ ĐKXĐ: \(\left\{{}\begin{matrix}3x+15\ne0\\9-4x\ge0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ne-5\\x\le\frac{9}{4}\end{matrix}\right.\)
b/ ĐKXĐ: \(\left\{{}\begin{matrix}x^3\ne0\\2x+8>0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ne0\\x>-4\end{matrix}\right.\)