3. \(\sqrt{x+2}\) - \(\sqrt{x^2-4}\)= 0 đkxđ : \(\left\{{}\begin{matrix}x\ge-2\\x^2-4\ge0\end{matrix}\right.\) \(\Leftrightarrow\) \(\left\{{}\begin{matrix}x\ge-2\\(x-2)(x+2)\ge0\end{matrix}\right.\)
\(\Leftrightarrow\) {-2} \(\cup\) { 2,+ \(\infty\)}
3. \(\sqrt{x+2}\) - \(\sqrt{x^2-4}\)= 0
\(\Leftrightarrow\) 3. \(\sqrt{x+2}\) - \(\sqrt{(x-2)(x+2)}\) = 0
\(\Leftrightarrow\) \(\sqrt{x+2}\).(3- \(\sqrt{x-2}\)) = 0
\(\Leftrightarrow\) \(\left[{}\begin{matrix}x=-2\\3-\sqrt{x-2}=0\end{matrix}\right.\)
\(\Leftrightarrow\) \(\left[{}\begin{matrix}x=-2\\x-2=9\end{matrix}\right.\)
\(\Leftrightarrow\) \(\left[{}\begin{matrix}x=-2\\x=11\end{matrix}\right.\)
=>\(\sqrt{9x+18}=\sqrt{x^2-4}\)
=>x^2-4=9x+18 và 9x+18>=0
=>x=2 hoặc x=-2
