# Systems of equations

If you're solving several equations in several variables
and you want a single solution that satisfies all of the equations at once,
then you're solving a *system* of equations.
Generally, you should have the same number of equations as variables,
and this should remain true
as you go through the process of solving the system.
So as you apply the techniques to solve equations
(such as substitution and addition-elimination),
then each new equation should always replace one of the old equations,
so that the total number of equations doesn't change.
The exceptions are for *dependent* systems of equations,
where eventually one of your equations becomes always true or always false.
If it becomes always true, then you throw it out,
and from then on, you have fewer equations.
If it becomes always false, then you throw the whole system out;
it is *inconsistent* and has no solutions.
Otherwise, you keep
the same number of equations and the same number of variables
―that is, even if some individual equations have fewer variables,
the system as a whole should keep the same number of variables―
until the system is solved.

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