使用 TikZ 注释 align* 方程

Question

我正在尝试将我的教授的讲课笔记数字化。

如何在方程式中添加箭头和注释？我知道可以使用 TikZ 来完成，但我才刚刚开始学习……所以我还不知道该怎么做。

以下是需要组装的不同部件。

begin{align*}
    L &= T - V \\
    &= \frac{1}{2} m \sum_{n=1}^N \dot{q}_n^2 - \frac{1}{2} \kappa \sum_{n=1}^N (q_{n+1} - q_n)^2
\end{align*}

\begin{equation*}
    \dot{x}_n (t) = \dot{q}_n (t)
\end{equation*}

It's only the difference in the displacements of the masses from their equilibrium position that contributes to the potential energy
\begin{align*}
    \big( x_{n+1} (t) - x_n (t) \big) - \epsilon &= ( n + 1 ) \epsilon + q_{n+1} (t) - n \epsilon - q_n (t) - \epsilon \\
    &= q_{n+1} (t) - q_n (t)
\end{align*}

Answer 1

谢谢@Andrew Stacey感谢您的建议！我使用以下说明成功了这一页和这一页\epsilon. （请注意，为了简单起见，我还用 替换了a）

{%
\begin{textblock*}{2in}(0.88in,1.25in)%
\begin{minipage}[h!]{2in}
    \begin{equation*}
        \tikzmarknode{xdot}{\boxed{\dot{x}_n (t) = \dot{q}_n (t)}}
    \end{equation*}
\end{minipage}%
\end{textblock*}%
}

{%
\begin{textblock*}{3in}(2.7in,1.1in)%
\begin{minipage}[h!]{3in}
    \noindent It's only the difference in the displacements of the masses from their equilibrium position that contributes to the potential energy
\begin{align*}
    \big( x_{n+1} (t) - x_n (t) \big) - a &= ( n + 1 ) \epsilon + q_{n+1} (t) \\
    &\quad - n a - q_n (t) - a \\
    &= q_{n+1} (t) - q_n (t)
\end{align*}
\end{minipage}%
\end{textblock*}%
}

\noindent The Lagrangian of this system is
\begin{align*}
    L &= T - V \\
    &= \frac{1}{2} m \sum_{n=1}^N \tikzmarknode{qdot}{\dot{q}_n^2} - \frac{1}{2} \kappa \sum_{n=1}^N \underbrace{ {(q_{n+1} - q_n)}^2 }
\end{align*}

\begin{tikzpicture}[overlay, remember picture]
    \draw[overlay,->] (xdot) -- (qdot.south);
\end{tikzpicture}