About this video
Sal solves P^2 = 0.81. Because a negative number squared is also positive, P is plus or minus the square root of 0.81. Knowing 9 × 9 = 81, he notices 0.9 × 0.9 has two decimal places and equals 0.81, so P = 0.9 or P = -0.9.
You'll be able to
- solve an equation like P^2 = 0.81 and find both solutions.
- use a known square (like 9^2 = 81) and decimal places to find the square root of a decimal.
Key words
- principal root
- The positive square root, shown by the radical symbol on its own.
- plus or minus (±)
- A sign showing two answers: one positive and one negative.
Transcript
- Let's see if we can solve the equation P squared is equal to 0.81. So how could we think about this? Well one thing we could do is we could say, look if P squared is equal to 0.81, another way of expressing this is, that well, that means that P is going to be equal to the positive or negative square root of 0.81.
Remember if we just wrote the square root symbol here, that means the principal root, or just the positive square root. But here P could be positive or negative, because if you square it, if you square even a negative number, you're still going to get a positive value.
So we could write that P is equal to the plus or minus square root of 0.81, which kind of helps us, it's another way of expressing the same, the same, equation. But still, what could P be? In your brain, you might immediately say, well okay, you know if this was P squared is equal to 81, I kinda know what's going on.
Because I know that nine times nine is equal to 81. Or we could write that nine squared is equal to 81, or we could write that nine is equal to the principal root of 81. These are all, I guess, saying the same truth about the universe, but what about 0.81? Well 0.81 has two digits behind, to the right of the decimal and so if I were to multiply something that has one digit to the right of the decimal times itself, I'm gonna have something with two digits to the right of the decimal.
And so what happens if I take, instead of nine squared, what happens if I take 0.9 squared? Let me try that out. Zero, I'm gonna use a different color. So let's say I took 0.9 squared. 0.9 squared, well that's going to be 0.9 times 0.9, which is going to be equal to?
Well nine times nine is 81, and I have one, two, numbers to the right of the decimal, so I'm gonna have two numbers to the right of the decimal in the product. So one, two. So that indeed is equal to 0.81. In fact we could write 0.81 as 0.9 squared. So we could write this, we could write that P is equal to the plus or minus, the square root of, instead of writing 0.81, I could write that as 0.9 squared.
In fact I could also write that as negative 0.9 squared. Cause if you put a negative here and a negative here, it's still not going to change the value. A negative times a negative is going to be a positive. I could, actually I would have put a negative there, which would have implied a negative here and a negative there.
So either of those are going to be true. But it's going to work out for us because we are taking the positive and negative square root. So this is going to be, P is going to be equal to plus or minus 0.9. Plus or minus 0.9, or we could write it that P is equal to 0.9, or P could be equal to negative 0.9.
And you can verify that, you would square either of these things, you get 0.81.
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