About this video
The video factors 25x^4 - 30x^2 + 9 by noticing that the first and last terms are perfect squares, then checking that the middle term equals two times the product of their square roots. Because the middle term is negative, it uses -3, giving (5x^2 - 3)^2.
You'll be able to
- recognise when the first and last terms of a trinomial are perfect squares.
- check the middle term is 2 times the product of the square roots.
- choose the correct sign and write the trinomial as a perfect square.
Key words
- Perfect square
- A number or expression that equals something squared, like 25x^4 = (5x^2)^2.
- Binomial
- An expression with two terms, such as 5x^2 - 3.
- Perfect square trinomial
- A three-term expression that equals a binomial squared.
Transcript
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.
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