`ĐKXĐ:{(x ne 2),(x ne 5):}`
\(\dfrac{x^2-25}{2x-4}.\dfrac{4}{5-x}\\ =\dfrac{\left(x-5\right)\left(x+5\right)}{2\left(x-2\right)}.\dfrac{4}{5-x}\\ =\dfrac{\left(5-x\right)\left(x+5\right)}{2\left(2-x\right)}.\dfrac{2.2}{5-x}\\ =\dfrac{2\left(x+5\right)}{2-x}\)
\(\dfrac{x^2-25}{2x-4}.\dfrac{4}{5-x}\)
\(=\dfrac{\left(x-5\right)\left(x+5\right)}{2\left(x-2\right)}.\dfrac{4}{5-x}\)
\(=\dfrac{\left(x-5\right)\left(x+5\right)}{2\left(x-2\right)}.\dfrac{-4}{x-5}\)
\(=\dfrac{x+5}{2\left(x-2\right)}.\dfrac{-4}{1}\)
\(=\dfrac{-4\left(x+5\right)}{2\left(x-2\right)}=\dfrac{-2\left(x+5\right)}{x-2}\)
