You shouldn't mix sans serif text with serif math. It looks odd. The eye gets distracted by things like that, tough mostly unconscious.
Better to use sans serif math as well, but be warned that not all symbols might be available in the font.
Code: Select all
\documentclass[10pt]{article}
\usepackage{amsmath}
\usepackage[cm]{sfmath}
\renewcommand*{\familydefault}{\sfdefault}
\begin{document}
Default:
{
This is some text with the variable K \textit{K} $K=10$ looking too large
}\par
Textstyle:
{%Do you note the different spacing here?
\everymath{\textstyle}%
This is some text with the variable $K=10$ still looking too large
}\par
Textstyle:
{
\everymath{\textstyle}
This is some text with the variable $K=10$ still looking too large
}\par
Scriptstyle:
{
\everymath{\scriptstyle}
This is some text with the variable $K=10$ looking too small
}
\end{document}
Over all, sans serif for longer texts is bad to read. Consider using a serif font in the whole document. If you have to stick to some odd regulations, you should stand up and give them a hint.
The smart way: Calm down and take a deep breath, read posts and provided links attentively, try to understand and ask if necessary.