Why is \textendash invalid in math mode?

Question

When editing the following document with Overleaf

\documentclass[a4paper, 12pt,fleqn]{article}
\usepackage{amsmath}
\usepackage{breqn}
\usepackage[utf8]{inputenc}
\usepackage{indentfirst}

\title{Exercice sur les équations}

\begin{document}

\maketitle

\begin{align*}
    12 + x &= 17 \\
    12 \boxed{- 12} + x &= 17 \boxed{- 12} \\
    x &= 5
\end{align*}

\begin{align*}
    3x – 2 &= 17 \\
    3x - 2 \boxed{+ 2} &= 17 \boxed{+ 2} \\
    3x &= 19
\end{align*}

\end{document}

I get the expected output

but there is a warning (at least I think this is a warning due to the yellow triangle)

Is this something serious?

(I apologize if this is obvious, I am back to LaTeX after 20 years trying to help my children with the formatting of their homework after seeing with disgust what they were about to send to the teacher (we are in France, confined at home))

Answer 1

The problem is that a character U+2013 (en-dash) is used instead of a minus sign U+2212 is used (or just a hyphen). This happens when people doesn't know the difference.

I also propose some enhancements to your code, adding \: in appropriate places to emulate the spacing that TeX would insert if \boxed were not here.

\documentclass[a4paper, 12pt,fleqn]{article}
\usepackage{amsmath}
%\usepackage{breqn}
%\usepackage[utf8]{inputenc}
\usepackage{indentfirst}

\title{Exercice sur les équations}

\begin{document}

\maketitle

\begin{align*}
    12 + x &= 17 \\
    12\: \boxed{-\:12} + x &= 17\: \boxed{-12} \\
    x &= 5
\end{align*}

\begin{align*}
    3x - 2 &= 17 \\
    3x - 2\: \boxed{+\:2} &= 17\:\boxed{+\:2} \\
    3x &= 19
\end{align*}

\end{document}