$a) M=\Big(\frac{x}{x^2-25}-\frac{x-5}{x^2+5x}\Big):\frac{2x-5}{x^2+5x}$
ĐK: $\left\{ \begin{array}{l}x^2-25\ne0\\x^2+5x\ne0\\2x-5\ne0\end{array} \right.$
$⇔\left\{ \begin{array}{l}x\ne{\pm5}\\x\ne0\\x\ne\frac{5}{2}\end{array} \right.$
$b) M=\Big(\frac{x}{x^2-25}-\frac{x-5}{x^2+5x}\Big):\frac{2x-5}{x^2+5x}$
$⇔M=\Big(\frac{x}{(x-5)(x+5)}-\frac{x-5}{x(x+5)}\Big):\frac{2x-5}{x(x+5)}$
$⇔M=\frac {x^2-(x-5)^2}{x(x-5)(x+5)}.\frac{x(x+5)}{2x-5}$
$⇔M=\frac {(x-x+5)(x+x-5)}{x-5}.\frac{1}{2x-5}$
$⇔M=\frac {5(2x-5)}{x-5}.\frac{1}{2x-5}$
$⇔M=\frac {5}{x-5}$
$c) M=\frac{5}{x-5}$
Tại x=2,5 $⇒M=\frac{5}{2,5-5}=-2$
