Math notes between equations

math-modenotes

Hi I was searching for a function that allows me to do what the picture shows, if someone could make a function like \underset{}_{}, where you put what you want to appear below the text, to make a function that allows me to put information pointing with an arrow to a place in the equation. Thank you for your help. Also, sorry for not having any code.

enter image description here

Best Answer

I would use tikzmark.

enter image description here

\documentclass{article}
\usepackage{tikz,amsmath}
\usetikzlibrary{tikzmark}
\usepackage{lipsum} %>>> for dummy text only
\begin{document}
\lipsum[1]
\vspace*{8mm}  % a gap for tikzmark later. Change as you wish

\[A(r)=\int_{V'}\dfrac{\mu_0}{4\pi}\dfrac{M(r')\times \hat{R} d\tau'}{R^2}\;
\tikzmarknode{e1}{=}
\;\dfrac{\mu_0}{4\pi}\int_{V'}M(r')\times\nabla'\left(\dfrac{1}{R}\right)d\tau'
\]
\begin{tikzpicture}[overlay,remember picture]
\draw[stealth-,cyan,thick] (e1)+(90:.2)--+(90:.7) node[above=-2mm]{$\dfrac{\hat R}{R^2}=\nabla'\left(\dfrac{1}{R}\right)$}; 
\end{tikzpicture}   
\lipsum[2]
\end{document}
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