Example 1  Solve .

Solution

In this equation the basic problem is the square root.  If that weren’t there we could do the problem.  The whole process that we’re going to go through here is set up to eliminate the square root.  However, as we will see, the steps that we’re going to take can actually cause problems for us.  So, let’s see how this all works.

Let’s notice that if we just square both sides we can make the square root go away.  Let’s do that and see what happens.

Upon squaring both sides we see that we get a factorable quadratic equation that gives us two solutions  and .

Now, for no apparent reason, let’s do something that we haven’t actually done since the section on solving linear equations.  Let’s check our answers.  Remember as well that we need to check the answers in the original equation!  That is very important.

Let’s first check  

So  is a solution.  Now let’s check .

We have a problem.  Recall that square roots are ALWAYS positive and so  does not work in the original equation.  One possibility here is that we made a mistake somewhere.  We can go back and look however and we’ll quickly see that we haven’t made a mistake. 

So, what is the deal?  Remember that our first step in the solution process was to square both sides.  Notice that if we plug  into the quadratic we solved it would in fact be a solution to that.  When we squared both sides of the equation we actually changed the equation and in the process introduced a solution that is not a solution to the original equation.

With these problems it is vitally important that you check your solutions as this will often happen.  When this does we only take the values that are actual solutions to the original equation.

So, the original equation had a single solution .

Example 2  Solve each of the following equations.

(a)    [Solution]

(b)    [Solution]

(c)    [Solution]

Solution

(a)  

In this case let’s notice that if we just square both sides we’re going to have problems.

Before discussing the problem we’ve got here let’s make sure you can the squaring that we did above since it will show up on occasion.  All that we did here was use the formula

with  and .  You will need to be able to do these because while this may not have worked here we will need to this kind of work in the next set of problems.

Now, just what is the problem with this?  Well recall that the point behind squaring both sides in the first problem was to eliminate the square root.  We haven’t done that.  There is still a square root in the problem and we’ve make the remainder of the problem messier as well.

So, what we’re going to need to do here is make sure that we’ve got a square root all by itself on one side of the equation before squaring.  Once that is done we can square both sides and the square root really will disappear.

Here is the correct way to do this problem.

As with the first example we will need to make sure and check both of these solutions.  Again, make sure that you check in the original equation.  Once we’ve square both sides we’ve changed the problem and so checking there won’t do us any good.  In fact checking there could well lead us into trouble.

First .

So, that is a solution.  Now .

So, as with the first example we worked there is in fact a single solution to the original equation, .

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(b)  

Okay, so we will again need to get the square root on one side by itself before squaring both sides.

So, we have a double root this time.  Let’s check it to see if it really is a solution to the original equation.

So,  isn’t a solution to the original equation.  Since this was the only possible solution, this means that there are no solutions to the original equation.  This doesn’t happen too often, but it does happen so don’t be surprised by it when it does.

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(c)  

This one will work the same as the previous two.

Let’s check these possible solutions start with .

So, that’s was a solution.  Now let’s check .

This was also a solution.

So, in this case we’ve now seen an example where both possible solutions are in fact solutions to the original equation as well.

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Example 3  Solve each of the following equations.

(a)    [Solution]

(b)    [Solution]

Solution

In both of these there are two square roots in the problem.  We will work these in basically the same manner however.  The first step is to get one of the square roots by itself on one side of the equation then square both sides.  At this point the process is different so we’ll see how to proceed from this point once we reach it in the first example.

(a)  

So, the first thing to do is get one of the square roots by itself.  It doesn’t matter which one we get by itself.  We’ll end up the same solution(s) in the end. 

Now, we still have a square root in the problem, but we have managed to eliminate one of them.  Not only that, but what we’ve got left here is identical to the examples we worked in the first part of this section.  Therefore, we will continue now work this problem as we did in the previous sets of examples.

Now, let’s check both possible solutions in the original equation.  We’ll start with  

So, the one is a solution.  Now let’s check .

So, they are both solutions to the original equation.

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(b)  

In this case we’ve already got a square root on one side by itself so we can go straight to squaring both sides.

Next, get the remaining square root back on one side by itself and square both sides again.

Now check both possible solutions starting with .

So, that wasn’t a solution.  Now let’s check .

It looks like in this case we’ve got a single solution, .

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