# How to typeset multiple equations center aligned but with single eq. number?

I want to typeset several equations with only one equation number vertically centered, and every equation horizontally center.

I know two options. One is `aligned` environment within `equation`, which handles equation number nicely, but the default alignment (without any `&`) is right alignment; and you know, you can't rely on `&` to force center alignment.

Another is `IEEEeqnarray` environment, which handles alignment nicely, but the equation numbering of which is somewhat awkward since numbering is processed on a line-to-line basis, as far as I know (if the number of equations is even, or equations take different vertical space, you will be screwed).

Any idea about this? Maybe I missed some functionality of `aligned` or `IEEEeqnarray`?

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Have you tried `gathered`? –  Andrew Swann Jan 15 '13 at 18:34
the package `mathtools` provides the `gathered` environment, which behaves like `aligned` (in that there is only one equation number generated), but otherwise behaves like `amsmath`'s `gather` (in centering every line separately).
`mathtools` loads `amsmath` automatically, and options to `amsmath` can be passed via specifying those options to `mathtools`.
As far as I know, `gathered` is already in `amsmath` –  egreg Jan 15 '13 at 18:43
@egreg Yes, it is. Most of time I work in `amsart`, which means I don't need extra package at all. –  KevinSayHi Jan 15 '13 at 18:53
@egreg -- aarrrggghhhh. i really ought to check the documentation before answering. i knew (at least) one of the "ed" forms was missing, and i mis-guessed. (there isn't any `multlined` in `amsmath`, but it's there in `mathtools`.) –  barbara beeton Jan 15 '13 at 19:13