[XeTeX] not a specifically xetex question: how to put a character flush to right in centered environment?

[Quang Minh Nguyen]

Hi,

I realize this is a basic question but I have a hard time finding the
answer to my inquiry.  I have a some centered environments (e.g.
center, equation, align) and would like to put a character, such as
the qed symbol, flush to the right.  How would one achieve that.

Thanks,
Minh
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Nguyễn Quang Minh || 阮光明