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I have the following block of TeX:

m_n^+ = \dfrac{1}{n}\sum\limits_{k=N+1}^{n} =& \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(a_k-L+L\right) \\
                                            <& \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(\dfrac{\epsilon}{2}+L\right) \\
                                            <& \dfrac{1}{n}\sum\limits_{k=1}^{n}\left(\dfrac{\epsilon}{2}+L\right) = \dfrac{\epsilon}{2}+L \\

How could I set a label for the last line in this block so that I may reference it later?

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Note your use of align's alignments are incorrect. You need &= and &<, not =& and <&. – Werner Dec 10 '12 at 6:52
@Werner Or ={}& and <{}& could be used to get the spacing right. Also the latest `\` should be removed to avoid an empty equation line. – Heiko Oberdiek Dec 10 '12 at 6:55
up vote 10 down vote accepted

You can either \tag the line in an (unnumbered) align* or \nonumber each equation you don't want in a (numbered) align. Here's a snapshot of both:

enter image description here

\usepackage{amsmath}% http://ctan.org/pkg/amsmath
m_n^+ = \dfrac{1}{n}\sum\limits_{k=N+1}^{n} &= \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(a_k-L+L\right) \\
                                            &< \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(\dfrac{\epsilon}{2}+L\right) \\
                                            &< \dfrac{1}{n}\sum\limits_{k=1}^{n}\left(\dfrac{\epsilon}{2}+L\right) = \dfrac{\epsilon}{2}+L 

m_n^+ = \dfrac{1}{n}\sum\limits_{k=N+1}^{n} &= \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(a_k-L+L\right) \nonumber \\
                                            &< \dfrac{1}{n}\sum\limits_{k=N+1}^{n}\left(\dfrac{\epsilon}{2}+L\right) \nonumber \\
                                            &< \dfrac{1}{n}\sum\limits_{k=1}^{n}\left(\dfrac{\epsilon}{2}+L\right) = \dfrac{\epsilon}{2}+L \label{myeq2}

The approach depends on the ease of use, or perhaps the size (number/quantity) of the set of the equations.

The former \tag-method uses the same counter (equation), although it requires manually stepping. In both instances, \label works as usual to capture the number for later use through \eqref.

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The best way to treat this, which is one equation in more lines is with split:

m_n^+ = \frac{1}{n}\sum_{k=N+1}^{n}
  &= \frac{1}{n}\sum_{k=N+1}^{n}(a_k-L+L) \\
  &< \frac{1}{n}\sum_{k=N+1}^{n}\left(\frac{\epsilon}{2}+L\right) \\
  &< \frac{1}{n}\sum_{k=1}^{n}\left(\frac{\epsilon}{2}+L\right) = \frac{\epsilon}{2}+L 


The default placement of the equation number is centered on the whole equation, as in the example; if one says


then the equation number will be aligned with the top or bottom row, according to whether numbers are at the left or the right margin.

Notice that I've removed the useless \limits commands (and also a \left-\right pair that's wrong). Also \dfrac can be \frac, because we are in a display.

enter image description here

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The [t] and [b] options work with split or they do not? Because I usually prefer the number to be at the last line... – yo' Dec 10 '12 at 9:47
@tohecz With the tbtags option the number will be at the last line, provided the equation number is on the right. It would have little sense putting the number at the last line with equation numbers at the left. – egreg Dec 10 '12 at 9:58

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