\(a,\left(x-\frac{1}{2}\right)^2=25=5^2=\left(-5\right)^2\)
\(=>\orbr{\begin{cases}x-\frac{1}{2}=5\\x-\frac{1}{2}=-5\end{cases}=>\orbr{\begin{cases}x=5+\frac{1}{2}=\frac{11}{2}\\x=-5+\frac{1}{2}=-\frac{9}{2}\end{cases}}}\)
\(b,\left(x-3\right)^2=81=9^2=\left(-9\right)^2\)
\(=>\orbr{\begin{cases}x-3=9\\x-3=-9\end{cases}=>\orbr{\begin{cases}x=9+3=12\\x=-9+3=-6\end{cases}}}\)