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## 8th grade

### Course: 8th grade > Unit 1

Lesson 4: Approximating irrational numbers- Approximating square roots
- Approximating square roots walk through
- Approximating square roots
- Comparing irrational numbers with radicals
- Comparing irrational numbers
- Approximating square roots to hundredths
- Comparing values with calculator
- Comparing irrational numbers with a calculator

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# Comparing irrational numbers with radicals

CCSS.Math:

Learn how to sort a bunch of numbers (4√2 2√3 3√2 √17 3√3 5) from least to greatest without using a calculator.

## Want to join the conversation?

- At3:05, why is it just answer 17 but at3:31, 5 is 25?(19 votes)
- He is squaring each number. So at3:05he squares the squared root of 17, the square root of 17x the square root of 17 equals 17. The square root of 17 is a number slightly bigger than 4, because 4x4 equals 16, so this is just a little bit more than that.

At3:31he square 5.

5x5=25

The concept is that if you square each number you can compare the numbers without the radical signs........(26 votes)

- how many times did he say square?(9 votes)
- It might be wrong, but I think it was 67 time!

***I counted it all, but it may be inaccurate!***(16 votes)

- Is this the only video about radicals? I haven't learned the basic stuff about radicals before this...(5 votes)
- This section has all the basic information about radicals:

https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:rational-exponents-radicals/x2f8bb11595b61c86:radicals/v/introduction-to-square-roots

Hope this helps!(7 votes)

- I’m confused on what exactly he’s doing. Is he squaring the square roots? Is that it?(2 votes)
- Yes, he is squaring the square roots and when you do this you always end up with the original number. For example: (sq.rt of 4)^2 is equal to 4. He is also squaring the factors being multiplied by the square roots(6 votes)

- how is √17 17?

I searched it up on google and it says the answer is 4.12310562562?(2 votes)- The reason for this is because when Sal squared all of the numbers, the square root of 17 (√17) when squared, is just the original number. So that means if you square √18 it would just be 18. I hoped this helped to answer your question.

If not, please visit the link below for additional information on this concept. Thanks, and have a good one.

https://www.khanacademy.org/math/cc-eighth-grade-math/cc-8th-numbers-operations/cc-8th-approximating-irrational-numbers/a/approximating-square-roots-guided-practice(3 votes)

- at1:44I don't understand how square root of 2 times square root of two is 2, since the square root of two is 1.something...(2 votes)
- If you have separate square roots, you can combine them, so √2*√2=√(2*2)=√4=2. It could also be seen in rational exponents. √2=2^(1/2) and anything multiplied by itself is just squaring it, so (2^(1/2))^2, power to power exponent rule is to multiply and 1/2*2=1.(3 votes)

- Why is 5 in the middle of 18 and 27(1 vote)
- Because the square of 5 is 25 and 25 is between 18 and 27.(4 votes)

- I don't know what a radical is. Help plz.(2 votes)
- Wait so 4√2 is that 4 x √2 or like 4√2 (like you know how you can cube/ square but instead there's a 4?) Please help ;-;(2 votes)
- If you start with √32, you get that equal to √16*√2=4√2. Generally, anytime two different things are next to each other (5x or 4√2 or 3(2x-5)) these all mean multiply.(2 votes)

- Natural numbers:

all the whole numbers except 0

Whole numbers:

all of the counting numbers (1, 2, 3, etc.) plus 0

Integers:

(can be positive or negative)

all of the whole numbers (1, 2, 3, etc.) plus all of their opposites (-1, -2, -3, etc.) and also 0

Rational numbers:

any number that can be expressed as a fraction of two integers (like 92, -56/3, √25, or any other number with a repeating or terminating decimal)

Irrational numbers:

all the numbers that can't be expressed as a fraction of two integers (like π, √7, or any other number with a non-repeating & non-terminating decimal)

Hope this helps!(2 votes)

## Video transcript

- I have six numbers here and you see that five
of them are irrational. They involve the square root
of a non-perfect square. Our goal in this video
is, without a calculator, see if we can sort these
numbers from least to greatest. And like always, pause this video and
see if you can do that. So I'll give you a hint. The hint is-- it's very hard without a calculator. Square root of two is gonna be one point
something, something. Square root of three is gonna be one point
something, something. How do we do this? We just have to realize
that, if I have some number, let's say I have some number
a that is greater than 0. And if we know that a is less than b, then a squared is going
to be less than b squared. If one positive number is less
than another positive number, then the square of this positive number is going to be less than
the square of that number. So one thing that we could
do when we are comparing all of these irrational
numbers that involve square roots of non-perfect squares, let's compare their squares. Because their squares are not going to be irrational numbers. It's going to be much easier to compare, and then we can order them. Because if we order the squares, then they'll tell us what happens if we order their square roots. What am I talking about? Well, I'm just gonna square each of these. So if I take this to the second power, this is going to be four
square roots of two, times four square roots of two. You can change the
order of multiplication. That's four times four times the square root of two
times the square root of two. Now, four times four is 16. Square root of two times
square root of two, well, that's just going to be two. So it's gonna be 16 times
two which is equal to 32. Now what about two square roots of three? Well, same idea. Let's square it, let's square it. And i'll do this one a little bit faster. So if we square two square roots of three, this is going to be two squared times square root of three squared. So it's going to be two squared times the square root of three squared. Well, two squared is going to be four. Square root of three squared
is going to be three. So this is going to be equal to 12. That's this thing squared. If this step seems a little bit confusing, if you have the product of
two things raised to a power, that's the same thing as raising each of them to that power, and then taking the product. And you can actually see,
I worked it out here, why that actually makes sense. Notice when I just changed
the order of multiplication you had four times four, or four squared, times square root of two squared,
which is going to be two. So let's keep doing that. So what is this value squared? It's gonna be three
squared, which is nine, times square root of two
squared, which is two. Nine times two is 18. What's the square root of 17 squared? That's just going to be seventeen. Do that in blue. This is just going to be 17. What is three square
roots of three squared? It's gonna be three
squared, which is nine, times square root of three squared. The square root of three
times the square root of three is three. So it's gonna be nine times three, or 27. And what is five squared? This is pretty straightforward. That's going to be 25. So let's order them
from least to greatest. Which of them, when I square it, gives me the smallest value? Compare 32 to 12 to 18 to 17 to 27 to 25. 12 is the smallest value. So if their square is the smallest, and these are all positive numbers, then this is going to
be the smallest value out of all of them. Let me write that first. Two square roots of three. So I've covered that one. Now what's next? Well, now I have this value. 17 is the next smallest
square, so its square root is going to be the next square root. So it's going to be two
square roots of three, then square root of 17. That is this one here. Then we go to 18. So if we look at its square root, with the numbers we were
originally trying to sort, that would be three square roots of two, three square roots of two, We got that one covered. Then the next one is gonna be 25, when we look at the squares. So the next value out of our original set, the next largest one, is going to be five. So then we get to five,
we've covered that one. Then the next one, let's see. We have 27 and 32 left. 27 is the next largest square,
so the next largest number out of the ones we care about, is three square roots of three. So three square roots of
three, we covered that one. And then we finish with-- This is the largest value,
four square roots of two. Four square roots of two. And we're done! That was pretty neat. Without a calculator, we were able to sort these irrational numbers, well, not all of them are irrational, but ones that involve the square root of something that is not a perfect square.