\(a,ĐK:a\ge\dfrac{2}{3}\\ b,ĐK:a\le\dfrac{5}{3}\\ c,ĐK:\dfrac{2a+1}{-4}\ge0\Leftrightarrow2a+1\le0\left(-4< 0\right)\Leftrightarrow a\le-\dfrac{1}{2}\\ d,ĐK:\dfrac{a-4}{7-a}\ge0\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a-4\ge0\\7-a>0\end{matrix}\right.\\\left\{{}\begin{matrix}a-4\le0\\7-a< 0\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a\ge4\\a< 7\end{matrix}\right.\\\left\{{}\begin{matrix}a\le4\\a>7\end{matrix}\right.\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}4\le a< 7\\a\in\varnothing\end{matrix}\right.\\ \Leftrightarrow4\le a< 7\)
