ĐKXĐ: \(x\ne\left\{-3;0;3\right\}\)
\(A=\left(\dfrac{x^2-3x}{3x\left(x-3\right)}+\dfrac{9}{3x\left(x-3\right)}\right):\left(\dfrac{x^2}{3\left(3-x\right)\left(3+x\right)}+\dfrac{3\left(3-x\right)}{3\left(3-x\right)\left(3+x\right)}\right)\)
\(=\left(\dfrac{x^2-3x+9}{3x\left(x-3\right)}\right):\left(\dfrac{x^2-3x+9}{3\left(3-x\right)\left(3+x\right)}\right)\)
\(=\dfrac{\left(x^2-3x+9\right)}{3x\left(x-3\right)}.\dfrac{3\left(3-x\right)\left(3+x\right)}{\left(x^2-3x+9\right)}\)
\(=-\dfrac{x+3}{x}=-1-\dfrac{3}{x}\)
\(A< -1\Rightarrow-1-\dfrac{3}{x}< -1\)
\(\Rightarrow\dfrac{3}{x}>0\Rightarrow x>0\)
Kết hợp ĐKXĐ \(\Rightarrow\left\{{}\begin{matrix}x>0\\x\ne3\end{matrix}\right.\)
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