# How can I write these expressions better?

I have no idea how to make it the right way. I have this warning message "Underfull \hbox (badness 10000)" (I have no idea what it means but I guess it has to do with the successive \hfill.)

\section{Dados utilizados para os cálculos da Torção}
$R=\SI{0.01}{\meter} \hfill t=\SI{0.0012}{\meter} \hfill r=\SI{0.0088}{\meter} \hfill b=\SI{0.18}{\meter}$\\\\
$T = P \hspace*{0.1cm} \cdot \hspace*{0.1cm} b$\hfill $E= \SI{70}{\giga\pascal}$\hfill $\nu=0.3$ \hfill $\alpha=\SI{45}{\degree}$
\\\\
$J=\frac{\pi}{2}\hspace*{0.1cm} \cdot \hspace*{0.1cm} (R^4-r^4)\implies J= \SI{6.29e-9}{\meter^4}$\hfill
$\tau_{xy}=\frac{T\hspace*{0.1cm} \cdot \hspace*{0.1cm} R}{J}$ \hfill $\gamma_{xy}=2\hspace*{0.1cm} \cdot \hspace*{0.1cm} \varepsilon_{zz}$ \vspace{1em}

\begin{align*}
&\varepsilon_{\alpha}=  \cancel{\varepsilon_{xx}\hspace*{0.1cm} \cdot \hspace*{0.1cm} \cos^2(\alpha)}\hspace*{0.1cm} + \hspace*{0.1cm}\cancel{\varepsilon_{zz}\hspace*{0.1cm} \cdot \hspace*{0.1cm} \cos^2(\alpha)}\hspace*{0.1cm} + \hspace*{0.1cm}\gamma_{xz}\hspace*{0.1cm} \cdot \hspace*{0.1cm} \sin(\alpha)\hspace*{0.1cm} \cdot \hspace*{0.1cm} \cos(\alpha) \iff\\
&\varepsilon_{\alpha}=2\hspace*{0.1cm} \cdot \hspace*{0.1cm} \varepsilon_{xy}\hspace*{0.1cm} \cdot \hspace*{0.1cm} \sin^2(\SI{45}{\degree}) \iff \varepsilon_{\alpha}=\varepsilon_{xy}
\end{align*}

$\varepsilon_{xy}=\frac{1+\nu}{E}\hspace*{0.1cm} \cdot \hspace*{0.1cm} \tau_{xy}$

• If you cross-post the same question on multiple sites, you should include links to all other versions of the question in every question. X-posting without explicit links has the risk of wasting both the time of users who might put time and effort into answering a post, which already has an answer elsewhere, as well as the time of users who might have the same problem but can't find the solutions you might have gotten on one of the other sites you x-posted your question Commented Jun 18 at 22:05
• Commented Jun 18 at 22:06
• never do \\\\  which is the cause of the warning tex.stackexchange.com/questions/334246/… also never have a blank line before align. Please provide an example that produces the warning, not just a fragment using undefined commands Commented Jun 18 at 22:07
• If you are using "siunitx" package, the "\SI" command changed to "\qty" approximately 10 years ago. Commented Jun 18 at 22:18

I suggest you use an alignat*{4} environment for the three-row expression. Do please get rid of all \hspace* directives and of most instances of \cdot. Definitely don't use \\\\; instead, use \\jot] -- if you think you need to insert a bit of additional vertical whitespace between rows. Don't be shy to use inline-fraction notation instead of \frac{...}{...}. And, unless you for some reason employ a ten-year-old (or older...) copy of the siunitx package, please write \qty instead of \SI. \documentclass{article} \usepackage[T1]{fontenc} \usepackage[portuguese]{babel} % or should that be 'brazilian'? \usepackage{amsmath} % for 'align*' and 'alignat*' environments \usepackage{siunitx} % for '\qty' macro \usepackage{cancel} % for '\cancel' macro \begin{document} \section{Dados utilizados para os cálculos da torção} \begin{alignat*}{4} R&=\qty{0.01}{\meter} &\qquad t&=\qty{0.0012}{\meter} & r&=\qty{0.0088}{\meter} & b&=\qty{0.18}{\meter} \\[\jot] % <-- note '\\[\jot]' % T&= P \cdot b & E&= \qty{70}{\giga\pascal} & \nu&=0.3 & \alpha &=\qty{45}{\degree}\\ % J&= \tfrac{\pi}{2} (R^4-r^4) &\implies J&= \qty{6.29e-9}{\meter^4} &\qquad \tau_{xy} &= T R/J &\qquad % <-- suggest you use inline-fraction notation \gamma_{xy} &= 2 \varepsilon_{zz} \end{alignat*} \begin{align*} \varepsilon_{\alpha} &= \cancel{\varepsilon_{xx} \cos^2(\alpha)} + \cancel{\varepsilon_{zz} \cos^2(\alpha)} + \gamma_{xz} \sin(\alpha) \cos(\alpha) \iff \\ \varepsilon_{\alpha} &= 2 \varepsilon_{xy} \sin^2(\qty{45}{\degree}) \iff \varepsilon_{\alpha}=\varepsilon_{xy} \end{align*} \[ \varepsilon_{xy} = \frac{1+\nu}{E} \, \tau_{xy}

\end{document}


First of all I'd get rid of all \cdot symbols that aren't idiomatic in mathematics. If you really want them, at least get rid of all \hspace*{0.1cm} instructions that just make the text harder to read.

I'd also avoid the double arrows \implies and \iff: there's no need for them.

The first group of formulas asks for alignedat; the second for aligned (so you can avoid the \iff by just aligning the equals signs). Since there's a final formula, all is enclosed in a gather* environment.

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[portuguese]{babel}
\usepackage{amsmath}
\usepackage{siunitx}
\usepackage{cancel}

\newcommand{\mcx}[3]{\multicolumn{#1}{#2}{$\displaystyle{}#3$}}

\begin{document}

\begin{gather*}
\begin{alignedat}{4}
b &=\qty{0.18}{\meter}
\\[1ex]
\alpha &= \qty{45}{\degree}
\\
J &\mcx{3}{l}{= \frac{\pi}{2}(R^4-r^4) = \qty{6.29e-9}{\meter^4}} &\qquad
\gamma_{xy} &= 2\varepsilon_{zz}
\end{alignedat}
\\[2ex]
\begin{aligned}
\varepsilon_{\alpha}
&= \cancel{\varepsilon_{xx} \cos^2(\alpha)}
+ \cancel{\varepsilon_{zz} \cos^2(\alpha)}
+ \gamma_{xz} \sin(\alpha) \cos(\alpha) \\
&= 2\varepsilon_{xy}\sin^2(\qty{45}{\degree}) \\
&= \varepsilon_{xy}
\end{aligned}
\\
\varepsilon_{xy}=\frac{1+\nu}{E}\tau_{xy}
\end{gather*}

\end{document}


Repeating \qquad after every & that jumps to the next right-aligned column may be annoying, but it avoids deciding which cell is the widest.

The only dirty trick I used is \multicolumn so we can span three columns in alignat.