Main content

### Course: Middle school math (India) > Unit 6

Lesson 3: Cube roots# Cube root of a non-perfect cube

Created by Sal Khan.

## Want to join the conversation?

- I am so very lost.

I have prime factorization down but this video leaves me utterly confused about how to find the cube root. The hints in the problems aren't very helpful. I do not understand how to go from prime factors to the cube root of non-perfect numbers. Please help!(24 votes)- You prime factorize the number then what you do is group the numbers appearing three times

like 7 in above video.group the other numbers appearing in times which are not multiples of three (2 and 5 in above example). multiply them and write them in the form cuberoot of

'multiplied number" (here 10 i.e 2*5).At last take the number you grouped first(number appearing three times) and write in form "X x cuberoot of y" where X=first grouped num..... y=second grouped num............ x=multiplied by(9 votes)

- I understand the method, but I spot two problems:

* won't different people get different answers depending upon which factors they try first?

* how do we know that the final surd form is the simplest?

Or is this just an illustration of factoring?

Thank you.(4 votes)- Well if you tried using different factors to start with, you will get the same answers eventually at the end when it is fully simplified. So in this example, you can start out by dividing by 7,5, or 49, for example, and the answer at the end for all three is still the same. You can tell when the final form is the simplest when all the numbers you have left are prime numbers or in other words they can't be broken down further (e.g. 2,3,5,7,11, etc.) Knowing what the prime numbers are comes with practice.(4 votes)

- What are imaginary numbers? How do they work?(3 votes)
- What the easiest way to do a cube root with a non-perfect cube(3 votes)
- Are there other ways (other than prime factorization) to find the cube root of a number?(1 vote)
- I think you can do the factor tree but not 100% so stick with this for now ...(2 votes)

- i am kinda lost please explain what sal means(3 votes)
- 1. Do the prime factorization of the number (Here:2,5,7,7,7). (I think Sal made a video about this.)

2. 7, 7 and 7 can be said as the cube root of 7*7*7.

3. 2 and 5 cannot be grouped so we can say it as the cube root of 10 (multiply them.)

4. Multiply them.(1 vote)

- I was wondering, what method can one use to calculate the cube root of a prime number without a calculator? for example I am supposed to know how to calculate 2 to the power 1/3 without using a calculator.(2 votes)
- Nope, these sort of things need a calculator. Without one your answer are normally expected stay 2^1/3. Now, if it wasn't a prime number you could do more with it.(2 votes)

- wait, i think i missed Exponent Properties... was that a video?(2 votes)
- At0:59, Sal has to do long division to try to figure out the factors. Is there an easier/ less time consuming way of doing this? Like tricks for looking at numbers and being able to skip some parts of this lengthy process?(2 votes)
- Sal could have skipped the first long division, or 1715/5, by simply factoring out 10 first. This gives him factors of 10 and 343, easily seen without long division. The 10 then factors down into the same 2 and 5 he started with. For the 343, however, I don't know of an easy way to factor that further without the long division.(1 vote)

- How to find cube root of a non cube prime number.Please help me(2 votes)

## Video transcript

Let's see if we can find
the cube root of 3,430. And if you're like me, it
doesn't jump out of your mind what number times that same
number times that same number-- if you have three
of those numbers and you were to
multiply them together-- would be equal to 3,430. So what I'm going to do is
to try to prime factorize this to find all the
prime factors of 3,430 and see if any of
those prime factors show up at least three times. And that'll help us with this. So 3,430-- it's clearly
divisible by 5 and 2, or it's divisible by 10. So let's do that. So first we can divide it by 2. It's 2 times-- let's see. 3,430 divided by 2 is 1,715. Then we can divide
it by 5, as well. We can factor 1,715
into 5 and-- let me do a little bit of long
division on the side here. So if I have 1,715, and I'm
going to divide it by 5. 5 doesn't go into 1. It goes into 17 three times. 3 times 5 is 15. Subtract, you get 2, and
then you bring down a 1. 5 goes into 21 four times. 4 times 5 is 20. Subtract. Bring down the 5. 5 goes into 15 three times,
so it goes exactly 343 times. So 1,715 can be factored
into 5 times 343. Now, 343 might not
jump out at you as a number that
is easy to factor. It's clearly an odd number,
so it won't be divisible by 2. Its digits add up to 10,
which is not divisible by 3. So this isn't going
to be divisible by 3. It's not going to
be divisible by 4, because it's not divisible by 2. It's not going to
be divisible by 5. If it wasn't
divisible by 3 or 2, it's not going to
be divisible by 6. And now we get to 7. Usually when you
see a nutty number like this that
doesn't seem to be divisible by a lot
of things, it's always a good idea to try
things like 7, 11, 13. Because those tend to construct
very interesting numbers. So let's see if this
is divisible by 7. So if I take 343 and if
I want to divide it by 7, 7 goes into 30-- it doesn't
go into 3-- 7 goes into 34 four times. 4 times 7 is 28. Subtract, 34 minus 28 is 6. Bring down a 3. 7 goes into 63 nine times. 9 times 7 is 63. Subtract. We don't have any remainder. And I forgot to do
that last step up here. 3 times 15 is 15. Subtract, no remainder. It went in exactly. So here, 343 can be
factored into 7 and 49. And 49 might jump out at you. It can be factored
into 7 times 7. So this is interesting. I can rewrite all of this
here-- the cube root of 3,430-- now as the cube
root of-- I'm just going to write it in its
factored form-- 2 times 5 times-- I could write
7 times 7 times 7, or I could write times
7 to the third power. That captures these three
7's right over here. I have three 7's, and then
I'm multiplying them together. So that's 7 to the third power. And from our
exponent properties, we know that this is the
exact same thing as the cube root of 2 times 5 times
the cube root-- so let me do that in
that same, just so we see what colors
we're dealing with. So the cube root of 2 times 5,
which is the cube root of 10, times the cube
root-- and I think you see where this is going--
of 7 to the third power. Keeping track of the
colors is the hard part. And the cube root of 10,
we just leave it as 10. We know the prime factorization
of 10 is 2 times 5, so you're not going to just
get a very simple integer answer here. You would get some
decimal answer here, but here you get a very
clear integer answer. The cube root of 7 to
the third, well, that's just going to be 7. So this is just going to be 7. So our entire thing simplifies. This is equal to 7 times
the cube root of 10. And this is about
as simplified as we can get just using
hand arithmetic. If you want to get
the exact number here, you're probably best
off using a calculator.