Main content
8th grade
Course: 8th grade > Unit 1
Lesson 2: Square roots & cube roots- Intro to square roots
- Square roots of perfect squares
- Square roots
- Intro to cube roots
- Cube roots
- Worked example: Cube root of a negative number
- Equations with square roots & cube roots
- Square root of decimal
- Roots of decimals & fractions
- Equations with square roots: decimals & fractions
- Dimensions of a cube from its volume
- Square and cube challenge
- Square roots review
- Cube roots review
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Cube roots review
CCSS.Math:
Review cube roots, and try some practice problems.
Cube roots
The cube root of a number is the factor that we multiply by itself three times to get that number.
The symbol for cube root is cube root of, end cube root .
Finding the cube root of a number is the opposite of cubing a number.
Example:
start color #7854ab, 3, end color #7854ab, times, start color #7854ab, 3, end color #7854ab, times, start color #7854ab, 3, end color #7854ab = start color #7854ab, 3, end color #7854ab, start superscript, start color #ff00af, 3, end color #ff00af, end superscript, equals, start color #1fab54, 27, end color #1fab54
So root, start index, start color #ff00af, 3, end color #ff00af, end index = start color #7854ab, 3, end color #7854ab
Want to learn more about finding cube roots? Check out this video.
Finding cube roots
If we can't figure out what factor multiplied by itself three times will result in the given number, we can make a factor tree.
Example:
Here is the factor tree for 64:
So the prime factorization of 64 is 2, times, 2, times, 2, times, 2, times, 2, times, 2.
We're looking for cube root of, 64, end cube root, so we want to split the prime factors into three identical groups.
Notice that we can rearrange the factors like so:
So left parenthesis, 2, times, 2, right parenthesis, cubed, equals, 4, cubed, equals, 64.
So cube root of, 64, end cube root is 4.
Practice
Want to try more problems like this? Check out this exercise:
Finding cube roots
Or this challenge exercise:
Equations with square and cube roots
Want to join the conversation?
How do you figure out large cube root questions without guessing and checking?(21 votes)- Sometimes what I do is rememebr simple pefect squares. For example, 4=64, 3=27.Sometimes the thing that works the best is just multiplying the number you are figure out by the given factor.(9 votes)
I get it now cuz when me teacher explains it she makes it really complicated(15 votes)
But (-2)*(-2)*(2) also equals 8.
So aren’t there then two values for the cubed root of 8: 2 and -2?(3 votes)
But -2 and 2 aren't the same number, so you aren't technically cubing it, since cubes are the SAME number multiplied three times. Hope this helped!(9 votes)
So if the question is the cubed root of 64, would that mean (8*8=64) = (64/3)?(3 votes)
No, because when you cube something, you multiply it by itself three times, 4*4*4=64(6 votes)
I don't understand the problem: Finding the cube root of 64 to the 3 power? Doesn't make sense.(3 votes)
The process of taking the cube root is the reverse of the process of taking a number to the 3 power. So these processes undo each other; therefore, the answer is just 64.
Have a blessed, wonderful day!(5 votes)
I was supposed to factor out 216 but when I started with 2, I ended up with 2,2,2,3,9 and that doesn't work. I only got the right answer when I started with six. Why does 2 not work?(3 votes)
2 works! Let's try it.
We factor out a 2 from 216. We get 108
We factor out a 2 from 108. We get 54
We factor out a 2 from 54. We get 27
We factor out a 3 from 27. we get 9
We factor out a 3 from 9. We get 3
We factor out a 3 from 3. We get 1
Answer- 2, 2, 2, 3, 3, 3
Check- 2 × 2 × 2 × 3 × 3 × 3 = 216!
And we got it righty right!
Perhaps you did the Check wrong.
Well you have to factor 9 also in your answer- 2,2,2,3,9
9 can be factored as 3 × 3
So your answer is written as 2, 2, 2, 3, 3, 3(4 votes)
I don’t understand their answer for the last question of the practice. They give no explanation. Why can’t -7 be a solution to plus or minus cube root of 343?
They put the plus and minus in front of the square root for 49 (which I understand) but why can’t you have it in front of the cube root of 343, why not? They don’t explain.(3 votes)
(-7)^3 = -343.
-7*-7*-7 = -343, if we multiply a negative by itself 3 times, you get negative.(3 votes)
- Good Question
How do you figure out large cube root questions without guessing and checking?(4 votes) 
i dont understand this, how do i do it? (i didnt learn this)(1 vote)
cube roots are inverses of cubic function, so if 3^3=27, the cube rott of 27=3. If you prime factor 27, you get 27=9*3=3*3*3, so on cube roots, you need three of the same number multiplied together which will come out as a single 3.
5^3=125, so cube root of (125)=cube root (5^3) = 5.(4 votes)
- I fill like some of the question is hard and some of them are you know easy and some of them are hard and they get you ready for what grade you going to(2 votes)