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.
I would use tikzmark.
\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}
\newcommand as the usual syntax of TeX/LaTeX. However, I think that is not so convenient as you expect (try and see!) I like the hand-made, manual controls such as \vspace*{8mm}, etc
– Black Mild
Jan 03 '22 at 10:31
Rather than cram too much visual information into what is already a lengthy derivation, I'd employ \shortintertext{...} and \intertext{...} directives to intersperse the explations with the actual steps of the derivation. That way, you're giving your readers some visual "breathers" between the steps. If you feel it's necessary or beneficial, you can still use colors (say, blue) to differentiate the explanations visually from the actual formulas.
\documentclass{article}
\usepackage{mathtools,bm,xcolor}
\begin{document}
\begin{align}
\bm{A}(\bm{r})
&= \int\limits_{V'} \frac{\mu_0}{4\pi} \frac{\bm{M}(\bm{r}') \times \widehat{\bm{R}}, d\tau'}{R^2}\
\shortintertext{Employ \textcolor{blue}{$\displaystyle\frac{\widehat{\bm{R}}}{R^2}=\bm{\nabla}'\biggl(\frac{1}{R}\biggr)$}:}
&= \frac{\mu_0}{4\pi} \int\limits_{V'} \bm{M}(\bm{r}')\times \bm{\nabla}'\biggl(\frac{1}{R}\biggr) , d\tau'\
\intertext{Employ \textcolor{blue}{$\displaystyle \bm{M}(\bm{r}')\times \bm{\nabla}'\biggl(\frac{1}{R}\biggr) =
\frac{\bm{\nabla}'\times \bm{M}}{R}-\bm{\nabla}'\times\biggl(\frac{\bm{M}}{R}\biggr)$}:}
&= \frac{\mu_0}{4\pi}\int\limits_{V'} \frac{\bm{\nabla}'\times \bm{M}}{R} ,d\tau' +
\frac{\mu_0}{4\pi}\int\limits_{V'}\biggl[-\bm{\nabla}'\times\biggl(\frac{\bm{R}}{R} \biggr) \biggr],d\tau'\
\intertext{Employ \dots}
\end{align}
\end{document}
witharrows. – F. Pantigny Jan 03 '22 at 11:40witharrowsis a LaTeX package I have written which provides a way to add indications about a calculus. It's available on CTAN. – F. Pantigny Jan 03 '22 at 13:10