You could "nudge" the subscript to the left, i.e., place it closer to the closing parenthesis, by writing _{\!\!2}
(two negative thinspaces) or _{\!2}
(one negative thinspace) instead of just _2
. Even better, though, would be not to use \left
and \right
to auto-size the parentheses to begin with. In fact, \left
and \right
may be the root cause of the problem, since the parentheses that are produced by \left(
and \right)
are needlessly large. Consider the following screenshot:

The first row is produced by your code, i.e., by \left(\frac{1}{2}A + B\right)_2
. The second row shows the effect of nudging the subscript to the left via _{\!\!2}
. In the third row, \Bigl
and \Bigr
are used instead of \left
and \right
; here, much less "nudging" is required.
The final row uses \tfrac
("text style \frac
") to create a smaller, but still perfectly readable fraction term along with \bigl
and \bigr
. Now, no nudging is required at all. I would argue that the expression in the final row is the most readable of the four. :-)
\documentclass{article}
\usepackage{amsmath} % for 'gather*' environment
\begin{document}
\begin{gather*}
\left(\frac{1}{2}A + B\right)_2 \\
\left(\frac{1}{2}A + B\right)_{\!\!2}\\
\Bigl(\frac{1}{2}A+B\Bigr)_{\!2}\\
\bigl(\tfrac{1}{2}A+B\bigr)_2
\end{gather*}
\end{document}
\left
and\right
. They add nothing and actually do harm. If you really need larger parentheses, for this case use\bigl(
and\bigr)
._2
use_{\!2}
or_{\!\!2}
. Also, while code snippets are useful in explanations, it is always best to compose a fully compilable MWE that illustrates the problem including the\documentclass
and the appropriate packages so that those trying to help don't have to recreate it.\left
and\ right
bad?