In org-mode, how do I include other .org files from a (main) .org file?

emacs, org-mode

Solution

Use `#+setupfile: /path/to/config.org` documented here.

Problem

I want to place some customization codes in a separate file, and include it from other files later. For example, file `config.org` has a single line `#+MATHJAX: align:"left" mathml:t`. How do I include it in another .org file `abc.org` so that the net-effect is exactly same as I write that `#+MATHJAX` line directly in abc.org?

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