Math & Science ⇒ align* and equation referencing

[jcress410]

I use align* to display longer sequences of equations (for homework assignments, specifically) and often I'd like to be able to set up a reference for an equation which will be used in a number of places later in the problem.

The use I'm thinking of here is when solving a problem and an identity for a variable is found (x = abc) that identity is plugged in later in the problem. But, if later in the problem the identity is found to be wrong (x actually is ab^2c or whatever) I'm forced to manually replace the identity elsewhere.

The only solution I can think of is using \newcommand{\} for every identity, this solution seems inelegant.

Is there a better way?