a) ĐKXĐ: \(\begin{cases}x\ne0\\x\ne-5\end{cases}\)
b) A = \(\frac{5x-50-\left(x-5\right)\left(2x+10\right)-x\left(x^2+2x\right)}{2x^2+10x}\) = \(\frac{-x^3-4x^2+5x}{2x^2+10x}\) = \(\frac{-x^2-4x+5}{2x+10}\)
= \(\frac{\left(1-x\right)\left(x+5\right)}{2\left(x+5\right)}\) =\(\frac{1-x}{2}\)
c) Để A = 3 => \(\frac{1-x}{2}\) = 3 =>1 - x = 6 => x = - 7(t/m ĐKXĐ)