Gdtyjr

If you only want some of the lines in an align environment to not be numbered, just put nonumber before the end characters on each of the lines you don't want to have numbers. Otherwise, use align* as JCL suggested so that none of the lines will be numbered.

As a sidenote, the mathtools package, which makes some improvements on amsmath, provides a way of labeling only those equations that are referenced in the text.

documentclassarticle 
usepackagemathtools 
mathtoolssetshowonlyrefs 
begindocument 
begingather 
 a = b labeleq1 \ 
 c = d labeleq2 
endgather 
Some text, eqrefeq2. 
enddocument

In this case, only the second of the equations, c = d, is numbered, as that is the only one being referenced in the text.

Notes

• three compiler passes are required for the references to show up properly


• this does not work with cleveref, but the cleveref manual mentions another package, autonum, that performs the same task, yet is compatible with cleveref.


• the manual of the present version (dated 2012/05/10) notes two bugs, one can cause that the number is printed close to, or on top of, the equation, the other relates to ntheorem. Refer to the manual for details.

Note whilst the above answers are relevant to the question, a lot of people include equations in LaTeX using the beginequation which works without explicitly including the amsmath package in a LaTeX document. However when one attempts to the use beginequation* directive (to omit numbering) an error is generated. Thus in this case it should be pointed out that one needs to explicitly import the amsmath package:

usepackageamsmath

If you use this:

beginequation
...
endequation

try this instead:

beginequation*
...
endequation*

"nonumber" sometimes does not work for me. Another possibility is to do:

begincenter
$ P(A cup B) = sum_omega in A cup B P(omega) = sum_omega in A
P(omega) + sum_omega in B P(omega) = P(A) + P(B) $
endcenter

documentclassarticle
usepackageamsmath
begindocument
 Solve these problems
 beginalign*
 1. ;; & x= y+z & 2.;; &x^n= y^n+z^n \ 
 3.;; & x= y+z & 4.;; & x^n= y^n+z^n \ 
 endalign*

 bf This is some sample text we will use to create a somewhat longer text
 spanning a few lines.

 beginalign*
 5.;; & x= y+z & 6.;; & x^n= y^n+z^n\ 
 endalign*
enddocument

This gives required output.