Main content

### Course: College Algebra > Unit 13

Lesson 3: Evaluating exponents and radicals- Evaluating fractional exponents
- Evaluating fractional exponents: negative unit-fraction
- Evaluating fractional exponents: fractional base
- Evaluating quotient of fractional exponents
- Evaluating mixed radicals and exponents
- Evaluate radical expressions challenge

© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Evaluating quotient of fractional exponents

Sal simplifies the complicated expression 256^(4/7) / 2^(4/7) until he finds that the expression is equal to 16.

## Want to join the conversation?

- I am not sure why one cant subtract the exponents while we are dividing. Given:

256^(4/7) / 2^(4/7) Why does the rule of subtracting exponents(4/7 - 4/7) in this situation fail? Thank you(18 votes)- Your bases have to be the same for the rule of subtracting exponents.(12 votes)

- What about the exponent is a irrational number?Like we describe 2^0.5=sqrt(2) , but how do we define 2^sqrt(2)?(8 votes)
- Doesn't simplifying the fraction (dividing 256 by 2 to reach 128) violate the PEMDAS rule whereby operation of exponent precedes that of division or multiplication?(6 votes)
- There is a property of exponents that tells us that having a fraction raised to an exponent is the same as having both the numerator and denominator individually raised to the exponent.

For example: (1/2)^3 = 1^3/2^3

The problem in the video is both the numerator and denominator with the same exponent. So, Sal uses this property exponents to bring the fraction back together, which allows him to then do the division.

Hope this helps.(9 votes)

- So 3.14 = 3/4?

I Dont Understand(3 votes)- No....

3.14 is a mixed number. The 3 is the whole number and the 0.14 is a fraction.

3.14 = 3 14/100 = 3 7/50.

I have no idea how you came up with 3/4.

Here is the lesson on converting decimals to fractions: https://www.khanacademy.org/math/arithmetic-home/arith-review-decimals#decimals-to-fractions

Hope this helps.(13 votes)

- I thought in the exponents rules, a^m / a^n would end up having the exponents minus each other out and in (256^4/7) / (2^4/7) would become 256^0 / 256^0 and therefore would equal 1/1 and then 1. How come it's not like this?(3 votes)
- All properties of exponents require that you have a common base. As you noted: a^m/a^n = a^(m-n), but the a's must be the same.

256 does not equal 2. So, there is no common base which is why your approach doesn't work.

Sal used the property of a^m/b^m = (a/b)^m. This let him divide 256/2, then apply the exponent to get to his answer of 16.

Alternatively, you can convert to a common base.

256 = 2^8

(2^8)^(4/7) = 2^(8*4/7) = 2^(32/7)

Then apply the property: a^m/a^n = a^(m-n)

2^(32/7) / 2^(4/7) = 2^(32/7-4/7) = 2^(28/7) = 2^4 or 16

Hope this helps.(8 votes)

- why do we need to know this when we could just use a calculator(6 votes)
- At1:54, what dos he mean by computationally intensive?(3 votes)
- A computation is just a mathematical calculation, so to say something is computationally intensive meaning that its hard to calculate. For instance, 1+1 is very easy to compute, so easy in fact you probably did it in your head without even thinking. Something like 128^4, or 53619 x 79863 would be considered hard to compute (without a calculator of course (unless perhaps you're a genius))(7 votes)

- Can't you subtract exponents when dividing? Thank You.(2 votes)
- Yes. If you have 2 values with a common base, you can subtract their exponents to do the division.

For example: 2^(5/4) / 2^(1/2) = 2^(5/4-1/2)

Since you are subtracting fractions, you need a common denominator: 2^(5/4-2/4) = 2^(3/4)

Hope this helps.(7 votes)

- hi. so i made the bases the same, and then subtracted the exponents due to division, and i arrived at the same answer. is the method that i used valid?

(2^8)^4/7 2^32/7

---------- = -------- = 2^28/7 = 2^4 = 16

2^4/7 2^4/7(3 votes) - I don't understand why 128 to the 1/7 power is 2 even though I do understand that 2 to the 7th power is 128...I just can't see the connection :((0 votes)
- A rational exponent (a fraction as an exponent) is just a different form of notation for radicals.

If you have 128^(1/7), the exponent of 1/7 means or is the same as 7th root(128). Both = 2.

Just like we have different symbols or ways to show multiplication or division, this is just another way to denote radicals.

Hope this helps.(6 votes)

## Video transcript

- [Voiceover] Let's see
if we can figure out what 256 to the four-sevenths power, divided by two to the
four-sevenths power is, and like always, pause the
video and see if you can figure this out. All right, let's work
through this together, and at first you might
find this kind of daunting. Especially when you see
something like two to the four-sevenths power or is that even, that's not going to be a
whole number, how do I, how do I do this, especially
without a calculator. And I should've said, do
this without a calculator. But then the key is to
see that we can use our exponent properties to
simplify this a little bit so that we can do this on paper. And the main property
that may jump out at you is if I have something, if I have, if I have x to the a power, over y to the a power, this is
the same thing as x over y, to the a power. And in our situation right over here, 256 would be x, two would be y, and then a is four-sevenths,
so we can rewrite this, this is going to be equal to this is equal to 256, over two, to the four-sevenths
power, and so this is nice. We're already able to simplify this, because we know 256
divided by two, is 128. So this is 128 to the four-sevenths power. Now this might also seem
a little bit difficult, how do I raise 128 to a fractional power? But we just have to remind ourselves, this is the same thing, this
is the same thing as 128 to the one-seventh power. Then raised to the fourth power. We could also view it the
other way around, we could say that this is also 128 to the fourth, to the fourth power, and
then raise that to the one-seventh, but
multiplying 128 four times, that's going to be very
computationally intensive, and then you have to find
the seventh root of that. That seems pretty difficult,
so we don't want to go in that way, but if we can
get the smaller number first, what is 128 to the one-seventh power? Then that might be easier to
raise to the fourth power. Now when you look at this,
and knowing that probably, the question writer in this
case, I'm the person who presented it with you is,
telling you that you're not going to use a calculator is,
it's a pretty good clue that, all right, this is probably going to be a, this is probably going to
be something that I can figure out on my own, and you might recognize
128 as a power of two, and maybe two to the seventh
is 128, and we can verify that. So let's see, two to the first is two. Four, eight, 16, 32, 64, 128. Two times two is four, times two is eight, times two is 16, times
two is 32, times 2 is 64, times two is 128. So, two to the seventh power is equal to 128, or another way of saying this exact same thing is that 128, 128 is equal to or 128 to the one-seventh power, is equal to two. Or you could even say
that the seventh root, the seventh root of 128, is equal, is equal to two. So, we can simplify this. This is two, so our whole
expression is now just two to the fourth power. Well, that's just two times two, times two, times two. So, that's two to the fourth power. Two to the fourth power, which is just going to be equal to 16. That's two, times two, times two, times two, right over there. And so we're done! This crazy,
complicated-looking expression, it is simplified to 16.