$${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{12}}}^{{\mathtt{2}}} = {{\mathtt{13}}}^{{\mathtt{2}}}$$
$${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{12}}}^{{\mathtt{2}}} = {{\mathtt{13}}}^{{\mathtt{2}}} \Rightarrow \left\{ \begin{array}{l}{\mathtt{a}} = -{\mathtt{5}}\\
{\mathtt{a}} = {\mathtt{5}}\\
\end{array} \right\}$$
$${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{b}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}} \Rightarrow \left\{ \begin{array}{l}{\mathtt{a}} = {\mathtt{\,-\,}}{\sqrt{{{\mathtt{c}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{{\mathtt{b}}}^{{\mathtt{2}}}}}\\
{\mathtt{a}} = {\sqrt{{{\mathtt{c}}}^{{\mathtt{2}}}{\mathtt{\,-\,}}{{\mathtt{b}}}^{{\mathtt{2}}}}}\\
\end{array} \right\}$$
