1: Ta có: \(\sqrt{81x^2}=6\)
\(\Leftrightarrow\left|9x\right|=6\)
\(\Leftrightarrow\left[{}\begin{matrix}9x=6\\9x=-6\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{2}{3}\\x=-\dfrac{2}{3}\end{matrix}\right.\)
2: Ta có: \(\sqrt{4x^2}+\sqrt{9x^2}=10\)
\(\Leftrightarrow2\left|x\right|+3\left|x\right|=10\)
\(\Leftrightarrow\left|x\right|=2\)
hay \(\left[{}\begin{matrix}x=2\\x=-2\end{matrix}\right.\)
3: Ta có: \(\sqrt{36x^2}-4=0\)
\(\Leftrightarrow\left|6x\right|=4\)
\(\Leftrightarrow\left[{}\begin{matrix}6x=4\\6x=-4\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{2}{3}\\x=-\dfrac{2}{3}\end{matrix}\right.\)
