Tip:
Highlight text to annotate it
X
We need to factor 25x to the fourth minus 30x
squared plus 9.
And this looks really daunting because we have something to
the fourth power here.
And then the middle term is to the second power.
But there's something about this that
might pop out at you.
And the thing that pops out at me at least is that 25 is a
perfect square, x to the fourth is a perfect square, so
25x to the fourth is a perfect square.
And 9 is also perfect square, so maybe this is the square of
some binomial.
And to confirm it, this center term has to be two times the
product of the terms that you're squaring on either end.
Let me explain that a little bit better
So, 25x to the fourth, that is the same thing as 5x squared
squared, right?
So it's a perfect square.
9 is the exact same thing as, well, it could be plus or
minus 3 squared.
It could be either one.
Now, what is 30x squared?
What happens if we take 5 times plus or minus 3?
So remember, this needs to be two times the product of
what's inside the square, or the square root of this and
the square root of that.
Given that there's a negative sign here and 5 is positive,
we want to take the negative 3, right?
That's the only way we're going to get a negative over
there, so let's just try it with negative 3.
So what is what is 2 times 5x squared times negative 3?
What is this?
Well, 2 times 5x squared is 10x squared times negative 3.
It is equal to negative 30x squared.
We know that this is a perfect square.
So we can just rewrite this as this is equal to 5x squared--
let me do it in the same color.
5x squared minus 3 times 5x squared minus 3.
And we saw in the last video why this works.
And if you want to verify it for yourself,
multiply this out.
You will get 25x to the fourth minus 30x squared plus 9.