a) \(\left(x+3\right)^2=4\)
\(\Leftrightarrow\left[{}\begin{matrix}\left(x+3\right)^2=2^2\\\left(x+3\right)^2=\left(-2\right)^2\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x+3=2\\x+3=-2\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-1\\x=-5\end{matrix}\right.\)
Vậy \(x\in\left\{-1;-5\right\}\)
b) \(\left(x-1\right)^2-81=0\)
\(\Leftrightarrow\left(x-1\right)^2=81\)
\(\Leftrightarrow\left[{}\begin{matrix}\left(x-1\right)^2=9^2\\\left(x-1\right)^2=\left(-9\right)^2\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x-1=9\\x-1=-9\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=10\\x=-8\end{matrix}\right.\)
Vậy \(x\in\left\{10;-8\right\}\)
