Thu gọn các hệ điều kiện sau:

a/ \(\left\{{}\begin{matrix}x\in(-1;3]\\x\in\left(-\infty;2\right)\cup\left(4;+\infty\right)\end{matrix}\right.\)

b/\(\left\{{}\begin{matrix}8\le x\le30\\\left[{}\begin{matrix}x< 10\\x\ge25\end{matrix}\right.\end{matrix}\right.\)

c/\(\left\{{}\begin{matrix}\left[{}\begin{matrix}x< -2\\x>4\end{matrix}\right.\\\left[{}\begin{matrix}x< -5\\x\ge7\end{matrix}\right.\end{matrix}\right.\)