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Studying for a test? Prepare with these 10 lessons on Quadratics.

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# Worked example: Rewriting expressions by completing the square

Video transcript

- [Voiceover] Let's
see if we can take this quadratic expression here, X
squared plus 16 X plus nine and write it in this form. You might be saying, hey Sal, why do I even need to worry about this? And one, it is just
good algebraic practice to be able to manipulate things, but as we'll see in the
future, what we're about to do is called completing the square. It's a really valuable
technique for solving quadratics and it's actually the basis for the proof of the quadratic formula which
you'll learn in the future. So it's actually a pretty
interesting technique. So how do we write this in this form? Well one way to think about
is if we expanded this X plus A squared, we know if
we square X plus A it would be X squared plus two A X plus A squared, and then you still have that
plus B, right over there. So one way to think about it is, let's take this expression, this X squared plus 16 X plus nine, and I'm just gonna write
it with a few spaces in it. X squared plus 16 X and then plus nine, just like that. And so, if we say alright,
we have an X squared here. We have an X squared here. If we say that two A X is
the same thing as that, then what's A going to be? So this is two A times X. Well, that means two A is 16 or that A is equal to 8. And so if I want to have
an A squared over here, well if A is eight, I
would add an eight squared which would be a 64. Well I can't just add numbers
willy nilly to an expression without changing the
value of an expression so if don't want to change
the value of the expression, I still need to subtract 64. So notice, all that I have done now, is I just took our original expression and I added 64 and I subtracted 64, so I have not changed the
value of that expression. But what was valuable about me doing that, is now this first part of the expression, this part right over here, it fits the pattern of a perfect square quadratic right over here. We have X squared plus
two A X, where A is 8, plus A squared, 64. Once again, how did I get 64? I took half of the 16 and I
squared it to get to the 64. And so the stuff that I just squared off, this is going to be X plus eight squared. X plus eight, squared. Once again I know that because
A is eight, A is eight, so this is X plus eight squared, and then all of this business
on the right hand side. What is nine minus 64? Well 64 minus nine is 55, so this is going to be negative 55. So minus 55, and we're done. We've written this
expression in this form, and what's also called completing the square.