sai thì thôi nhó :q
`a)Đ k : x ne +- 3`
\(A=\sqrt{\dfrac{1}{x^2-6x+9}}-\dfrac{6}{x^2-9}\)
\(A=\dfrac{\sqrt{1}}{\sqrt{\left(x-3\right)^2}}-\dfrac{6}{\left(x+3\right)\left(x-3\right)}=\dfrac{1}{\left|x-3\right|}-\dfrac{6}{\left(x+3\right)\left(x-3\right)}\)
\(A=\dfrac{\left(x+3\right)\left(x-3\right)-\left(x-3\right)6}{\left(x-3\right)^2\left(x+3\right)}=\dfrac{\left(x+3-6\right)\left(x-3\right)}{\left(x-3\right)^2\left(x+3\right)}\)
\(A=\dfrac{\left(x-3\right)^2}{\left(x-3\right)^2\left(x+3\right)}=\dfrac{1}{x+3}\)
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