How to center the tag for a multiline diagram?

I have a diagram, say five lines, the tag (equation numbering) is in the first line, this is not I want. How to force the tag stay in the middle line? Thanks in advance!

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You could use the amsmath package, which provides an aligned environment:
\begin{aligned} \Sigma &= a + b + c + d\\ & + e + f + g + h\\ & + i + j + k + l \end{aligned}

In the same package there's also gathered which doesn't use the & sign for alignment -- depending on your equation this may be of more use to you. –  Ant Aug 2 '11 at 9:55
And split is preferred to aligned. –  Leo Liu Aug 2 '11 at 14:35