# How to add a subscript character inside an equation

I want to add something like FA (the force at point A.), I've been doing it this way:
$F$<sub>$A$</sub> but inside an equation starting with 2 dollar signs(I want it to be written on a separate line Like:
$$F&lt;sub>A&lt;/sub>$$ this is causing some troubles, and it seems that it couldn't be done this way, so how can I do it?

Underscore. F_A in TeX is $F_A$. For longer strings use {} groups like F_{10}: $F_{10}$
The tool we use for math rendering is called MathJax. It provides a close simulation of LaTeX math-mode. The subscript operator is _, so you write F_A.