Code: Select all
\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot]{geometry}
\usepackage{nccmath,amsmath}
\usepackage{amsfonts}
\usepackage{mathtools}
\begin{document}
Blabla bla:
\begin{equation}
\sin^2 \vartheta = \sin^2 \! \vartheta
\end{equation}
\end{document}
\!
(a negative space) before the argument almost solves the spacing issue, but it's not exactly "right". What would you do in such a situation?