\(a,ĐKXĐ:\left\{{}\begin{matrix}8-4x\ge0\\5x-10\ne0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x\le2\\x\ne2\end{matrix}\right.\Leftrightarrow x< 2\\ b,ĐKXĐ:\left\{{}\begin{matrix}\dfrac{-3}{x+2}\ge0\\x^2+2\ge0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x+2< 0\left(-3< 0;x+2\ne0\right)\\x\in R\left(x^2+2\ge2>0\right)\end{matrix}\right.\Leftrightarrow x< -2\)