It seems simple enough: solve or .

So what do we tell our students to do? We might say things like “with an equation we must do the same thing to both sides of the equation (other than multiply both sides by zero)” and with an inequality, “we have to remember to reverse the inequality if we, say, multiply both sides by a negative number or if we take the reciprocal”.

And, of course, we need to check afterwards to see if we haven’t improperly expanded the solution set.

But what is really going on? A moment’s thought will reveal that what we are doing is applying the appropriate function to both sides of the equation/inequality.

And, depending on what we are doing, we want to ensure that the function that we are applying is one-to-one and taking note if the function is increasing or decreasing in the event we are solving an inequality.

Example: Now the standard way is to subtract from both sides (which is a one to one function..subtract constant number) which yields . Now we might say “square both sides” to obtain but only works. But the function that does that, the “squaring function”, is NOT one to one. Think of it this way: if we have and we then square both sides we now have which has the original solution and . So in our example, the extraneous solution occurs because but .

If you want to have more fun, try a function that isn’t even close to being one to one; e. g. solve by taking the sine of both sides. 🙂

(yes, I know, NO ONE would want to do that).

As far as inequalities: the idea is to remember that if one applies a one-to-one function on both sides, one should note if the function is increasing or decreasing.

Example: . We did the switch when the function that we applied ( was decreasing.)

Example: solving requires that we use the conditional definition for absolute value and reconcile our two answers: and which leads to the union of or

The fun starts when the function that we apply is neither decreasing nor increasing. Example: Needless to say, the function, by itself, is inadequate without adjusting for periodicity.

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[…] more advanced discussion can be found here; basically, it boils down to: when you do the same process to both sides of an equation, be careful […]

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