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.

Here's an example that I originally prepared for my College Algebra courses, where you are sometimes expected to solve systems of equations with 3 variables instead of only 2. (But the basic techniques are the same.) So this is more complicated than anything that you'll have to do in this course, but you can see how I kept track of everything without losing my place.

Or download it: WebM format, Ogg Vorbis format, MPEG-4 format.

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