2

I have the following group of equations:

\begin{align*}
    T' & =kn \\
    b' & =\tau n\\
    n' & =-\tau b-kT
\end{align*}

I want to put these equations into one box and to put the description "Formule di Frenet" centered on the right.

In other words, I want to extend the following result valid for one equation:

    \begin{equation*}
        \boxed{r:\alpha(t_0)+\Braket{\alpha'(t_0)}} \qquad \emph{retta tangente alla curva $\alpha$ nel punto $\alpha(t_0)$}
    \end{equation*}
1

Use aligned for the \boxed group.

\documentclass{article}
\usepackage{amsmath,braket}
\begin{document}
    \begin{equation*}
        \boxed{r:\alpha(t_0)+\Braket{\alpha'(t_0)}} 
    \qquad \emph{retta tangente alla curva $\alpha$ nel punto $\alpha(t_0)$}
    \end{equation*}

    \begin{equation*}
        \boxed{\begin{aligned}
    T' & =kn \\
    b' & =\tau n\\
    n' & =-\tau b-kT
\end{aligned}} \qquad \emph{Formule di Frenet}
    \end{equation*}
\end{document}

enter image description here

1

For fun, another possibility, adapted from a code in the documentation of empheq, which produces a shaded box with title:

\documentclass{article}
\usepackage[svgnames]{xcolor}
\usepackage{empheq}

\colorlet{shadecolor}{FloralWhite!60}
\colorlet{titleshade}{OliveDrab!15}
\newsavebox{\mysaveboxM} % M for math
\newsavebox{\mysaveboxT} % T for text
\newcommand*\Garybox[2][Example]{%
\sbox{\mysaveboxM}{#2}%
\sbox{\mysaveboxT}{\fcolorbox{black}{titleshade}{\enspace#1\enspace}}%
\sbox{\mysaveboxM}{%
\parbox[b][\ht\mysaveboxM+.5\ht\mysaveboxT+.5\dp\mysaveboxT][b]{%
\wd\mysaveboxM}{#2}%
}%
\sbox{\mysaveboxM}{%
\fcolorbox{black}{shadecolor}{%
\makebox[\linewidth-10em]{\usebox{\mysaveboxM}}%
}%
}%
\usebox{\mysaveboxM}%
\makebox[0pt][r]{%
\makebox[\wd\mysaveboxM][c]{%
\raisebox{\ht\mysaveboxM-0.5\ht\mysaveboxT
+0.5\dp\mysaveboxT-0.5\fboxrule}{\usebox{\mysaveboxT}}%
}%
}%
}%

\begin{document}

\begin{subequations}
\begin{empheq}[box={\Garybox[Formule di Frenet]}]{align}
    T' & =kn \\
    b' & =\tau n\\
    n' & =-\tau b-kT
\end{empheq}
\end{subequations}

\end{document}

enter image description here

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