Difference between revisions of "Exponentiation"
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Similarly, the exponentiation is defined as the repetition of multiplication. For example, writing out <math>3\cdot 3\cdot 3\cdot 3\cdot 3</math> can get boring fast, so we define the exponential function to express this in a much more compact form so that the preceeding example can be written as <math>3^5</math> (read 3 to the 5th or 3 to the 5 power). What this means is that we are multiplying 3 by itself 5 times. The base is 3 (what is repeatedly multiplied) and the exponent (or power) is 5 (the number of times to repeat multiplication). | Similarly, the exponentiation is defined as the repetition of multiplication. For example, writing out <math>3\cdot 3\cdot 3\cdot 3\cdot 3</math> can get boring fast, so we define the exponential function to express this in a much more compact form so that the preceeding example can be written as <math>3^5</math> (read 3 to the 5th or 3 to the 5 power). What this means is that we are multiplying 3 by itself 5 times. The base is 3 (what is repeatedly multiplied) and the exponent (or power) is 5 (the number of times to repeat multiplication). | ||
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Our definition of exponentiation makes sense if the exponent is a positive integer. How about negative integers such as <math>2^{-4}</math>? How do we multiply 2 by itself -4 times!? Let's think about what a negative sign means a little more. When we append a negative sign to a number (say 4, for example), we are basically saying go four units in the ''opposite'' direction. We want to do the opposite of multiplication four times. In other words, we want to divide by 2 four times. Therefore, <math> 2^{-4}=\frac 1{2^4}.</math> | Our definition of exponentiation makes sense if the exponent is a positive integer. How about negative integers such as <math>2^{-4}</math>? How do we multiply 2 by itself -4 times!? Let's think about what a negative sign means a little more. When we append a negative sign to a number (say 4, for example), we are basically saying go four units in the ''opposite'' direction. We want to do the opposite of multiplication four times. In other words, we want to divide by 2 four times. Therefore, <math> 2^{-4}=\frac 1{2^4}.</math> | ||
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== Basic Properties == | == Basic Properties == | ||
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** From the property of same base multiplication, <math>b^x \cdot b^0 = b^{x+0} = b^x</math>, so dividing both sides by <math>b^x</math> results in <math>b^0 = 1</math>. | ** From the property of same base multiplication, <math>b^x \cdot b^0 = b^{x+0} = b^x</math>, so dividing both sides by <math>b^x</math> results in <math>b^0 = 1</math>. | ||
** <math>0^0</math> is undefined. | ** <math>0^0</math> is undefined. | ||
− | * <math> b^{-x} = \frac{1}{b^x}</math> | + | * <math> b^{-x} = \frac{1}{b^x}</math> if <math>b \neq 0</math> |
** From the property of same base multiplication and zero exponent, <math>b^x \cdot b^{-x} = b^{x + -x} = b^0 = 1</math>, so dividing both sides by <math>b^x</math> results in <math>b^{-x} = \frac{1}{b^x}</math>. | ** From the property of same base multiplication and zero exponent, <math>b^x \cdot b^{-x} = b^{x + -x} = b^0 = 1</math>, so dividing both sides by <math>b^x</math> results in <math>b^{-x} = \frac{1}{b^x}</math>. | ||
* <math>\frac{b^x}{b^y} = b^{x-y}</math> | * <math>\frac{b^x}{b^y} = b^{x-y}</math> |
Latest revision as of 16:25, 10 August 2020
Exponentiation is an arithmetic operation, just like addition, multiplication, etc. It is often written in the form , where
is the exponent (or power) and
is the base.
In the order of operations, it is the second operation performed if a equation has parentheses or the first one performed when there is no parentheses.
Introduction
To understand how exponents arise, let's first review how we can build multiplication from addition. Let's say we wanted to capture the notion of "the amount equal to 3, ten times." We could write this out as , but this gets burdensome quickly: if we wanted to capture the idea of "the amount equal to two hundred 3s." Thus, we define the multiplication function, usually denoted
or
, such that
where there are 200 threes in the sum. This process (actually an inductive definition) defines the operation of "multiplication by positive integers." We can then extend the notion of multiplication to non-integers.
Similarly, the exponentiation is defined as the repetition of multiplication. For example, writing out can get boring fast, so we define the exponential function to express this in a much more compact form so that the preceeding example can be written as
(read 3 to the 5th or 3 to the 5 power). What this means is that we are multiplying 3 by itself 5 times. The base is 3 (what is repeatedly multiplied) and the exponent (or power) is 5 (the number of times to repeat multiplication).
Our definition of exponentiation makes sense if the exponent is a positive integer. How about negative integers such as ? How do we multiply 2 by itself -4 times!? Let's think about what a negative sign means a little more. When we append a negative sign to a number (say 4, for example), we are basically saying go four units in the opposite direction. We want to do the opposite of multiplication four times. In other words, we want to divide by 2 four times. Therefore,
Basic Properties
Listed below are some important properties of exponents (with explanations and notes):
- We are multiplying
together
times and then
times, so in total, we are multiplying
together
times.
- We are multiplying
if
- From the property of same base multiplication,
, so dividing both sides by
results in
.
is undefined.
- From the property of same base multiplication,
if
- From the property of same base multiplication and zero exponent,
, so dividing both sides by
results in
.
- From the property of same base multiplication and zero exponent,
- This results from using the property of same base multiplication and negative exponent property.
- We are multiplying
by itself
times, and because of the first property, we are multiplying
by itself
times.
- We are multiplying
- We are multiplying
together
times, which is the same as multiplying
together
times and
together
times.
- We are multiplying
Fractional Exponents
So far we discussed expressions with integer exponents. However, it is also possible to extend the exponential function to all non-integers.
How could we make sense of an expression like ? If you don't already know the answer, this is a good exercise; I recommend puzzling over it for awhile.
Answer: Hoping that property 1 will remain true when or
is a fraction, we see that
should (hopefully) be equal to
. Thus, we define
to be
, in order to make this be true.
For the time being, how to deal with other fractions in the exponent can be an exercise for the reader.
Hint: What would be? What about
? Do you notice anything?
Try to figure out -- how does it relate to
?
Problems
- Practice Problems on Alcumus
- General Positive Exponents (Prealgebra)
- Negative and Zero Exponents (Prealgebra)