Why isn’t math bold when set in bold context, like \textbf{Text $f_i$}?
The f isn’t bold because it’s set in math mode and in math f and f can be different things. E.g. f is a vector and f the length of the vector.
LaTeX has its own treatment for math fonts which can be affected by the user with commands like \mathbf
. But the math fonts don’t depend on the surrounding text font. You may have a look at Make mathfont respect the surrounding family, where you can find settings to get your desired behaviour. An image taken frome there to illustrate the result:
Please post complete documents not just fragments (so this isn't tested) but you want
\begin{document}
\textbf{\boldmath\textit{Les caractéristiques $f_i$}}
\end{document}
Just to mention all possibilities, there is also the bm
package with its \bm
command:
\documentclass[12pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\usepackage{bm}
\begin{document}
\textbf{\textit{Les caractéristiques $\bm{f_i}$}}
\end{document}