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## Pre-algebra

### Course: Pre-algebra > Unit 11

Lesson 3: Negative exponents# Negative exponent intuition

CCSS.Math:

Intuition on why a^-b = 1/(a^b) (and why a^0 =1). Created by Sal Khan.

## Want to join the conversation?

- What is understanding exponents useful for? can it ever be used in daily life?(42 votes)
- You can use them for taxes, material management, and funds.(15 votes)

- Why do we even use exponents; when will we ever even use them in life?(9 votes)
- Here are some real life applications of exponents.

1. Calculations of areas (including surface areas) and volumes of objects

2. Calculations of distances in situations involving right triangles (Pythagorean Theorem)

3. Calculations involving loans or savings accounts, when interest is compounded

4. Calculations of probabilities of compound events

5. Calculations pertaining to motions of objects (for example, the height of an object thrown in the air as a function of time)

6. Expressing very small or very large measurements in science (for example, using scientific notation to express the mass of an electron or the mass of a planet)

7. Geometric Brownian motion model of the behavior of stock prices

8. Calculations in statistics, such as variance and standard deviation

9. Population growth or decay models

Have a blessed, wonderful day!(34 votes)

- Could somebody explain going backwards with exponents? It's a little bit difficult to understand.(7 votes)
- Think of this pattern:

2^3=8

2^2=4

2^1=2

2^0=1

2^-1=1/2

2^-2=1/4

See how we have a pattern of dividing by two every time? So going down in exponents equates to dividing instead of multiplying!(16 votes)

- does anybody know what a and b are??(0 votes)
- a and b are variables that stand for any number.(36 votes)

- what is 0 to the 0th power(6 votes)
- it is undefined, since x^y as a function of 2 variables is not continuous at the origin(11 votes)

- this vid makes no sense(7 votes)
- Potato Quality XDD I understand this video is ancient.(6 votes)
- Just to make sure I understand this correctly, there is no set explanation as to why, say, 2^-1 equals 1/2? That is to say it is an arbitrary rule? I have recently been trying to understand logarithms and my many questions of exponents have flooded back to me, haha. Thank you in advance!(1 vote)
- There is a logic progression that shows this to be true. As an example, going backwards from 2^3 = 8, divide both sides by 2 gives 2^2 = 4, 2^1 = 2, 2^0 = 1. When we keep going, 2^-1=.5 = 1/2, 2^-2 = .25 = 1/4, etc.(5 votes)

- How to have more understanding in this?(3 votes)
- this video is older than me :O(3 votes)

## Video transcript

I have been asked for some
intuition as to why, let's say, a to the minus b is equal
to 1 over a to the b. And before I give you the
intuition, I want you to just realize that this
really is a definition. I don't know. The inventor of mathematics
wasn't one person. It was, you know, a
convention that arose. But they defined this, and they
defined this for the reasons that I'm going to show you. Well, what I'm going to show
you is one of the reasons, and then we'll see that this is a
good definition, because once you learned exponent rules, all
of the other exponent rules stay consistent for negative
exponents and when you raise something to the zero power. So let's take the
positive exponents. Those are pretty
intuitive, I think. So the positive exponents, so
you have a to the 1, a squared, a cubed, a to the fourth. What's a to the 1? a to the 1,
we said, is a, and then to get to a squared, what did we do? We multiplied by a, right? a squared is just a times a. And then to get to a
cubed, what did we do? We multiplied by a again. And then to get to a to the
fourth, what did we do? We multiplied by a again. Or the other way, you could
imagine, is when you decrease the exponent,
what are we doing? We are multiplying by
1/a, or dividing by a. And similarly, you decrease
again, you're dividing by a. And to go from a squared
to a to the first, you're dividing by a. So let's use this progression
to figure out what a to the 0 is. So this is the first hard one. So a to the 0. So you're the inventor, the
founding mother of mathematics, and you need to define
what a to the 0 is. And, you know, maybe
it's 17, maybe it's pi. I don't know. It's up to you to decide
what a to the 0 is. But wouldn't it be nice if a to
the 0 retained this pattern? That every time you decrease
the exponent, you're dividing by a, right? So if you're going from a to
the first to a to the zero, wouldn't it be nice if
we just divided by a? So let's do that. So if we go from a to the
first, which is just a, and divide by a, right, so we're
just going to go-- we're just going to divide it by a,
what is a divided by a? Well, it's just 1. So that's where the
definition-- or that's one of the intuitions behind why
something to the 0-th power is equal to 1. Because when you take that
number and you divide it by itself one more time,
you just get 1. So that's pretty reasonable,
but now let's go into the negative domain. So what should a to
the negative 1 equal? Well, once again, it's nice if
we can retain this pattern, where every time we decrease
the exponent we're dividing by a. So let's divide by
a again, so 1/a. So we're going to take a to
the 0 and divide it by a. a to the 0 is one, so
what's 1 divided by a? It's 1/a. Now, let's do it one more
time, and then I think you're going to get the pattern. Well, I think you probably
already got the pattern. What's a to the minus 2? Well, we want-- you know,
it'd be silly now to change this pattern. Every time we decrease the
exponent, we're dividing by a, so to go from a to the minus 1
to a to the minus 2, let's just divide by a again. And what do we get? If you take 1/2 and divide by
a, you get 1 over a squared. And you could just keep doing
this pattern all the way to the left, and you would get a
to the minus b is equal to 1 over a to the b. Hopefully, that gave you a
little intuition as to why-- well, first of all, you know,
the big mystery is, you know, something to the 0-th power,
why does that equal 1? First, keep in mind that
that's just a definition. Someone decided it should
be equal to 1, but they had a good reason. And their good reason
was they wanted to keep this pattern going. And that's the same reason
why they defined negative exponents in this way. And what's extra cool about it
is not only does it retain this pattern of when you decrease
exponents, you're dividing by a, or when you're increasing
exponents, you're multiplying by a, but as you'll see in the
exponent rules videos, all of the exponent rules hold. All of the exponent rules are
consistent with this definition of something to the 0-th power
and this definition of something to the
negative power. Hopefully, that didn't confuse
you and gave you a little bit of intuition and demystified
something that, frankly, is quite mystifying the
first time you learn it.