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Simplifying radical expressions: three variables

A worked example of simplifying the cube root of 27a²b⁵c³ using the properties of exponents. Created by Sal Khan and Monterey Institute for Technology and Education.

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  • piceratops seedling style avatar for user Simone
    does that mean that the I can't simplify the cube root of 3?
    (17 votes)
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    • leafers seedling style avatar for user Jayant Wakode
      we can not simplify cube root 3...but..we can find its value by method of logaritms
      say y=cube root3...taking log on both sides, log y =log(cuberoot 3)=(1/3)(log 3)
      =(1/3)*0.48=0.16....now, log y =0.16....now we take antilog on both sides,,,,
      y=antilog 0.16 = 1.4454.....so, cuberoot 3=1.4454
      (2 votes)
  • leaf green style avatar for user jasonsanctis
    (3^3) X ( b^3) X (c^3) = (3bc)^ 3
    which property of exponents is Sal referring to,
    is it : (a^m)^n = (a)^ mn...
    my memory has misplaced this property.....oh
    Also please guide me to the video which Sal is referring to .. i intend to review it..
    (6 votes)
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    • leaf blue style avatar for user Matthew Daly
      There's a separate rule that (a^m) * (b^m) = (ab)^m.

      If you think about it, it all comes down to basic principles. a^m is just m copies of a multiplied together, right? So a^m b^m is just a*a*...a*b*b...b, with n copies of each, so we can rearrange those terms using the commutative property to get that it is also a*b*a*b...*a*b, which is n copies of ab multiplied together or (ab)^n.
      (13 votes)
  • blobby green style avatar for user tyrece14
    i dont get how letters can be perfect or not perfect
    (3 votes)
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  • marcimus pink style avatar for user Marina Burandt
    What about bigger roots, like root 4? A problem in my book is: simplify ^4 of z^8. What would I do there?
    (5 votes)
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    • leaf green style avatar for user mahansen42
      Roots can be turned into fractional exponents. the n'th root can be re-written as an exponent of 1/n. So the 4th root of x is the same as x^(1/4). Once you get that, then you should be able to use properties of exponents to finish the problem.
      (5 votes)
  • old spice man green style avatar for user Pedro
    Hi!

    I would like to know why the cube root of a given number is equal to that same number to the power of 1/3,

    ³√x = x to the 1/3 power.

    Why 1/3? What makes it 1/3? That's something I'm having a little trouble understanding!

    Thanks in advance!

    Cheers!
    (3 votes)
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    • male robot hal style avatar for user Andrew M
      That's a good question. Take a look at this:

      We know that when you multiply numbers that have exponents, you add the exponents, right? So for example, 2^3 * 2^2 = 2^5. And likewise, 2^1 * 2^1 *2^1 = 2^3, which equals 8. Now let's try it with a variable for the exponent, where we are trying to find the cube root of 8 by raising 8 to some undetermined power:

      8^x * 8^x * 8^x = 8^1 = 8. What does x have to be? When I add up three x's, I have to get 1. 3x =1. x = 1/3.

      So 8^(1/3) is the cube root of 8.

      You can show the same thing using the rule that says (a^n)^m = a^(n*m)

      (8^(1/3))^3 = 8^(1/3*3) = 8^1.
      (7 votes)
  • male robot hal style avatar for user imad ali
    what about taking the absolute value here , is it not necessary in case of cube root?
    (2 votes)
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    • female robot grace style avatar for user HumXD
      i'm guessing you meant 'do we have to do like square root and take the positive root?'

      cube root is pretty much the opposite of taking the number to the power of 3

      so...
      2^3= 8
      cuberoot(8)=2

      -2^3 = -2*-2*-2 = -8
      cuberoot(-8)= -2

      if you get cuberoot(8), you have only 1 answer.
      (5 votes)
  • leaf green style avatar for user Sophie
    What if the constant was not a perfect cube? Like the number 24? How would you do that? Thanks
    (3 votes)
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  • leaf orange style avatar for user Trey Branch
    Is there any other way to get the right answer??
    (3 votes)
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  • leafers ultimate style avatar for user Tom
    Okay, I get this. Now what to do with the cube root of 9+4√5? According to wolfram and my calc it can be simplified to (3+√5)/2. I get the simplification steps wolfram offers, but I couldn't find the general way to simplify this kind of expressions. (Some other examples are the cube root of 40+11√13 which is (5+√13)/2, and the cube root of 10-6√3, which is 1-√3..)
    (3 votes)
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  • blobby green style avatar for user colinsoder
    Would this method work for higher roots, like fourth and fifth roots?
    (3 votes)
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Video transcript

We're asked to simplify the cube root of 27a squared times b to the fifth times c to the third power. And the goal, whenever you try to just simplify a cube root like this, is we want to look at the parts of this expression over here that are perfect cubes, that are something raised to the third power. Then we can take just the cube root of those, essentially taking them out of the radical sign, and then leaving everything else that is not a perfect cube inside of it. So let's see what we can do. So first of all, 27-- you may or may not already recognize this as a perfect cube. If you don't already recognize it, you can actually do a prime factorization and see it's a perfect cube. 27 is 3 times 9, and 9 is 3 times 3. So 27-- its prime factorization is 3 times 3 times 3. So it's the exact same thing as 3 to the third power. So let's rewrite this whole expression down here. But let's write it in terms of things that are perfect cubes and things that aren't. So 27 can be just rewritten as 3 to the third power. Then you have a squared-- clearly not a perfect cube. a to the third would have been. So we're just going to write this-- let me write it over here. We can switch the order here because we just have a bunch of things being multiplied by each other. So I'll write the a squared over here. b to the fifth is not a perfect cube by itself, but it can be expressed as the product of a perfect cube and another thing. b to the fifth is the exact same thing as b to the third power times b to the second power. If you want to see that explicitly, b to the fifth is b times b times b times b times b. So the first three are clearly b to the third power. And then you have b to the second power after it. So we can rewrite b to the fifth as the product of a perfect cube. So I'll write b to the third-- let me do that in that same purple color. So we have b to the third power over here. And then it's b to the third times b squared. So I'll write the b squared over here. And we're assuming we're going to multiply all of this stuff. And then finally, we have-- I'll do in blue-- c to the third power. Clearly, this is a perfect cube. It is c cubed. It is c to the third power. So I'll put it over here. So this is c to the third power. And of course, we still have that overarching radical sign. So we're still trying to take the cube root of all of this. And we know from our exponent properties, or we could say from our radical properties, that this is the exact same thing. That taking the cube root of all of these things is the same as taking the cube root of these individual factors and then multiplying them. So this is the same thing as the cube root-- and I could separate them out individually. Or I could say the cube root of 3 to the third b to the third c to the third. Actually, let's do it both ways. So I'll take them out separately. So this is the same thing as the cube root of 3 to the third times the cube root-- I'll write them all in. Let me color-code it so we don't get confused-- times the cube root of b to the third times the cube root of c to the third times the cube root-- and I'll just group these two guys together just because we're not going to be able to simplify it any more-- times the cube root of a squared b squared. I'll keep the colors consistent while we're trying to figure out what's what. And I could have said that this is times the cube root of a squared times the cube root of b squared, but that won't simplify anything, so I'll just leave these like this. And so we can look at these individually. The cube root of 3 to the third, or the cube root of 27-- well, that's clearly just going to be-- I want to do that in that yellow color-- this is clearly just going to be 3. 3 to the third power is 3 to the third power, or it's equal to 27. This term right over here, the cube root of b to the third-- well, that's just b. And the cube root of c to the third, well, that is clearly-- I want to do that in that-- that is clearly just c. So our whole expression has simplified to 3 times b times c times the cube root of a squared b squared. And we're done. And I just want to do one other thing, just because I did mention that I would do it. We could simplify it this way. Or we could recognize that this expression right over here can be written as 3bc to the third power. And if I take three things to the third power, and I'm multiplying it, that's the same thing as multiplying them first and then raising to the third power. It comes straight out of our exponent properties. And so we can rewrite this as the cube root of all of this times the cube root of a squared b squared. And so the cube root of all of this, of 3bc to the third power, well, that's just going to be 3bc, and then multiplied by the cube root of a squared b squared. I didn't take the trouble to color-code it this time, because we already figured out one way to solve it. But hopefully, that also makes sense. We could have done this either way. But the important thing is that we get that same answer.