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## Extraneous solutions

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# Equation that has a specific extraneous solution

CCSS Math: HSA.REI.A.2

## Video transcript

- [Voiceover] We're
asked, which value for D, and we see D in this equation here, makes X equals negative
three an extraneous solution for this radical equation? Squared of 3x plus 25
is equal to D plus 2x. And I encourage you to
pause the video and try to think about it on your own before we work through it together. Alright, now let's work
through this together. So the first thing that
just to remind ourselves is what is an extraneous solution? Well that's a solution that
we get or we think we get but it's really just a
by-product of how we solved it but isn't going to be an actual solution of our original equation. Now how do these extraneous
solutions pop up? Well, it pops up when you
take the square of both sides. So for this equation right
over here, to get rid of the radical, I'd want
to square both sides of it. If I square both sides, the
lefthand side will become 3x plus 25 and the righthand
side, if I square this, is going to be, what? It's going to be D squared
plus 4, 4dx plus X squared. So that's just squaring
both sides of this, but notice, there's actually
a different equation than this one that if you squared
both sides, you would also get this. What is that different equation? Well the different equation
is if you took the negative of one of these sides. So for example, if you had,
if you started the original equation, the negative
square root of 3x plus 25 plus 25, is equal to D plus 2x. You square both sides of this,
you still get this purple equation because you square a negative, you get a positive. So both of them, when
you square both sides, get us over here. And so when you solve
this purple equation, this is a quadratic right over here. You just rearrange it a little bit. You get into standard quadratic form, you'll get two solutions. And it turns out, one of
the solutions is going to be for this yellow equation
and one of the solutions is going to be for the purple equation. And if the solution that
is for the purple equation, is going to be an extraneous solution for the yellow equation. This is actually not
going to be a solution for the yellow equation. So when they say, "Which
value for D makes X equals "negative three and extraneous solution "for this yellow equation?" That's the same thing as saying, "What value of D makes
X equals negative three "a solution for this?" So a solution for this. If it's a solution for this,
it's going to be an extraneous solution for that 'cause these
are two different equations. We're taking the negative of just one side of this equation to get this one. If you took the negative
of both sides of this and that becomes the same
thing 'cause you could multiply both sides of an
equation times a negative value. So a solution for this, which
is equivalently a solution which is equivalent to a
solution, if I, instead of putting the negative on the lefthand
side, if I multiply the righthand side by the negative. But anyway, let's think
about which value for D makes X equals negative
three a solution for this? Well, let's substitute X
equals negative three here and then we just have to solve for D. If X equals negative three,
this is going to be negative the square root of three
times negative three is negative nine, plus 25 is equal to D. Two times negative three is negative six. So D minus 6, and so now we can square both sides of, we can square, actually let's do it this way. We can, I don't wanna
square both sides 'cause we lose some information. It's going to be the negative
square root of negative nine plus 25 is 16, is equal to D minus 6, so this is going to be
equal to negative four. Principle over 16 is four. We have the negative out front, is equal to D minus six and
then add six to both sides. You get two is equal
to, two is equal to D. So if D is equal to two here, if D is equal to two, then
a solution to this purple equation is going to be
X equals negative three. And so that would be an
extraneous solution because if X equals negative three
satisfies this over here, it's definitely gonna
satisfy this over here, but it's not going to
satisfy this up here. And you could verify this if this is equal to two, try out X equals negative three. You're gonna get on the lefthand side, you're going to get 16
and on the righthand side, you're gonna get two minus six which is equal to negative four. Two minus six which is negative four, so this does not work out. X equals negative three
is not a solution to this, but it is a solution for
this and it is a solution to this quadratic right over here. So D equals two makes
X equal negative three, an extraneous solution for this equation.