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Wprowadzenie do pierwiastków kwadratowych

Transkrypcja filmu video (w języku angielskim)
- [Voiceover] If you're watching a movie and someone is attempting to do fancy mathematics on a chalkboard, you'll almost always see a symbol that looks like this. This radical symbol. And this is used to show the square root and we'll see other types of roots as well, but your question is, well, what does this thing actually mean? And now that we know a little bit about exponents, we'll see that the square root symbol or the root symbol or the radical is not so hard to understand. So, let's start with an example. So, we know that three to the second power is what? Three squared is what? Well, that's the same thing as three times three and that's going to be equal to nine. But what if we went the other way around? What if we started with the nine, and we said, well, what times itself is equal to nine? We already know that answer is three, but how could we use a symbol that tells us that? So, as you can imagine, that symbol is going to be the radical here. So, we could write the square root of nine, and when you look at this way, you say, okay, what squared is equal to nine? And you would say, well, this is going to be equal to, this is going to be equal to, three. And I want you to really look at these two equations right over here, because this is the essence of the square root symbol. If you say the square root of nine, you're saying what times itself is equal to nine? And, well, that's going to be three. And three squared is equal to nine, I can do that again. I can do that many times. I can write four, four squared, is equal to 16. Well, what's the square root of 16 going to be? Well, it's going to be equal to four. Let me do it again. Actually, let me start with the square root. What is the square root of 25 going to be? Well, this is the number that times itself is going to be equal to 25 or the number, where if I were to square it, I'd get to 25. Well, what number is that, well, that's going to be equal to five. Why, because we know that five squared is equal to, five squared is equal to 25. Now, I know that there's a nagging feeling that some of you might be having, because if I were to take negative three, and square it, and square it I would also get positive nine, and the same thing if I were to take negative four and I were to square the whole thing, I would also get positive 16, or negative five, and if I square that I would also get positive 25. So, why couldn't this thing right over here, why can't this square root be positive three or negative three? Well, depending on who you talk to, that's actually a reasonable thing to think about. But when you see a radical symbol like this, people usually call this the principal root. Principal root. Principal, principal square root. Square root. And another way to think about it, it's the positive, this is going to be the positive square root. If someone wants the negative square root of nine, they might say something like this. They might say the negative, let me scroll up a little bit, they might say something like the negative square root of nine. Well, that's going to be equal to negative three. And what's interesting about this is, well, if you square both sides of this, of this equation, if you were to square both sides of this equation, what do you get? Well negative, anything negative squared becomes a positive. And then the square root of nine squared, well, that's just going to be nine. And on the right-hand side, negative three squared, well, negative three times negative three is positive nine. So, it all works out. Nine is equal, nine is equal to nine. And so this is an interesting thing, actually. Let me write this a little bit more algebraically now. If we were to write, if we were to write the principal root of nine is equal to x. This is, there's only one possible x here that satisfies it, because the standard convention, what most mathematicians have agreed to view this radical symbol as, is that this is a principal square root, this is the positive square root, so there's only one x here. There's only one x that would satisfy this, and that is x is equal to three. Now, if I were to write x squared is equal to nine, now, this is slightly different. X equals three definitely satisfies this. This could be x equals three, but the other thing, the other x that satisfies this is x could also be equal to negative three, 'cause negative three squared is also equal to nine. So, these two things, these two statements, are almost equivalent, although when you're looking at this one, there's two x's that satisfy this one, while there's only one x that satisfies this one, because this is a positive square root. If people wanted to write something equivalent where you would have two x's that could satisfy it, you might see something like this. Plus or minus square root of nine is equal to x, and now x could take on positive three or negative three.