Exponential functions change addition into multiplication. Different bases for exponentials produce different functions but they share similar characteristics. One base--a number we call e--is an especially good one.
2. Outline
Deļ¬nition of exponential functions
Properties of exponential Functions
The number e and the natural exponential function
Compound Interest
The number e
A limit
. . . . . .
4. Deļ¬nition
If a is a real number and n is a positive whole number, then
an = a Ā· a Ā· Ā· Ā· Ā· Ā· a
n factors
Examples
23 = 2 Ā· 2 Ā· 2 = 8
34 = 3 Ā· 3 Ā· 3 Ā· 3 = 81
(ā1)5 = (ā1)(ā1)(ā1)(ā1)(ā1) = ā1
. . . . . .
5. Fact
If a is a real number, then
ax+y = ax ay
ax
axāy = y
a
(ax )y = axy
(ab)x = ax bx
whenever all exponents are positive whole numbers.
. . . . . .
6. Fact
If a is a real number, then
ax+y = ax ay
ax
axāy = y
a
(ax )y = axy
(ab)x = ax bx
whenever all exponents are positive whole numbers.
Proof.
Check for yourself:
a x +y = a Ā· a Ā· Ā· Ā· Ā· Ā· a = a Ā· a Ā· Ā· Ā· Ā· Ā· a Ā· a Ā· a Ā· Ā· Ā· Ā· Ā· a = a x a y
x + y factors x factors y factors
. . . . . .
7. Letās be conventional
The desire that these properties remain true gives us
conventions for ax when x is not a positive whole number.
. . . . . .
8. Letās be conventional
The desire that these properties remain true gives us
conventions for ax when x is not a positive whole number.
For example:
a n = a n +0 = a n a 0
!
. . . . . .
9. Letās be conventional
The desire that these properties remain true gives us
conventions for ax when x is not a positive whole number.
For example:
a n = a n +0 = a n a 0
!
Deļ¬nition
If a Ģø= 0, we deļ¬ne a0 = 1.
. . . . . .
10. Letās be conventional
The desire that these properties remain true gives us
conventions for ax when x is not a positive whole number.
For example:
a n = a n +0 = a n a 0
!
Deļ¬nition
If a Ģø= 0, we deļ¬ne a0 = 1.
Notice 00 remains undeļ¬ned (as a limit form, itās
indeterminate).
. . . . . .
12. Conventions for negative exponents
If n ā„ 0, we want
an Ā· aān = an+(ān) = a0 = 1
!
Deļ¬nition
1
If n is a positive integer, we deļ¬ne aān = .
an
. . . . . .
13. Conventions for negative exponents
If n ā„ 0, we want
an Ā· aān = an+(ān) = a0 = 1
!
Deļ¬nition
1
If n is a positive integer, we deļ¬ne aān = .
an
Fact
1
The convention that aān = āworksā for negative n as well.
an
am
If m and n are any integers, then amān = n .
a
. . . . . .
15. Conventions for fractional exponents
If q is a positive integer, we want
(a1/q )q = a1 = a
!
Deļ¬nition ā
If q is a positive integer, we deļ¬ne a1/q = q
a. We must have
a ā„ 0 if q is even.
. . . . . .
16. Conventions for fractional exponents
If q is a positive integer, we want
(a1/q )q = a1 = a
!
Deļ¬nition ā
If q is a positive integer, we deļ¬ne a1/q = q
a. We must have
a ā„ 0 if q is even.
Fact
Now we can say ap/q = (a1/q )p without ambiguity
. . . . . .
18. Conventions for irrational powers
So ax is well-deļ¬ned if x is rational.
What about irrational powers?
Deļ¬nition
Let a > 0. Then
ax = lim ar
rāx
r rational
. . . . . .
19. Conventions for irrational powers
So ax is well-deļ¬ned if x is rational.
What about irrational powers?
Deļ¬nition
Let a > 0. Then
ax = lim ar
rāx
r rational
In other words, to approximate ax for irrational x, take r close to x
but rational and compute ar .
. . . . . .
29. Graphs of various exponential functions
y
.
yy = 213)x
. . = ((//2)x (1/3)x
y
. = . = (1/10)x. = 10x= 3x. = 2x
y y y
. y . = 1.5x
y
. = 1x
y
. x
.
. . . . . .
30. Outline
Deļ¬nition of exponential functions
Properties of exponential Functions
The number e and the natural exponential function
Compound Interest
The number e
A limit
. . . . . .
31. Properties of exponential Functions
Theorem
If a > 0 and a Ģø= 1, then f(x) = ax is a continuous function with
domain R and range (0, ā). In particular, ax > 0 for all x. If
a, b > 0 and x, y ā R, then
ax+y = ax ay
ax
axāy = y
a
(ax )y = axy
(ab)x = ax bx
Proof.
This is true for positive integer exponents by natural
deļ¬nition
Our conventional deļ¬nitions make these true for rational
exponents
Our limit deļ¬nition make these for irrational exponents, too
. . . . . .
32. Properties of exponential Functions
Theorem
If a > 0 and a Ģø= 1, then f(x) = ax is a continuous function with
domain R and range (0, ā). In particular, ax > 0 for all x. If
a, b > 0 and x, y ā R, then
ax+y = ax ay
ax
axāy = y negative exponents mean reciprocals.
a
(ax )y = axy
(ab)x = ax bx
Proof.
This is true for positive integer exponents by natural
deļ¬nition
Our conventional deļ¬nitions make these true for rational
exponents
Our limit deļ¬nition make these for irrational exponents, too
. . . . . .
33. Properties of exponential Functions
Theorem
If a > 0 and a Ģø= 1, then f(x) = ax is a continuous function with
domain R and range (0, ā). In particular, ax > 0 for all x. If
a, b > 0 and x, y ā R, then
ax+y = ax ay
ax
axāy = y negative exponents mean reciprocals.
a
(ax )y = axy fractional exponents mean roots
(ab)x = ax bx
Proof.
This is true for positive integer exponents by natural
deļ¬nition
Our conventional deļ¬nitions make these true for rational
exponents
Our limit deļ¬nition make these for irrational exponents, too
. . . . . .
39. Fact (Limits of exponential
functions) y
.
. = (= 2()1/3)x3)x
y . 1/=x(2/
y
y . . = (. /10)10x = 2x. =
y y = x . 3x y
y y
1 . =
If a > 1, then
lim ax = ā and
xāā
lim ax = 0
xāāā
If 0 < a < 1, then
lim ax = 0 and y
. =
xāā
lim ax = ā . x
.
xāāā
. . . . . .
40. Outline
Deļ¬nition of exponential functions
Properties of exponential Functions
The number e and the natural exponential function
Compound Interest
The number e
A limit
. . . . . .
41. Compounded Interest
Question
Suppose you save $100 at 10% annual interest, with interest
compounded once a year. How much do you have
After one year?
After two years?
after t years?
. . . . . .
42. Compounded Interest
Question
Suppose you save $100 at 10% annual interest, with interest
compounded once a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100 + 10% = $110
. . . . . .
43. Compounded Interest
Question
Suppose you save $100 at 10% annual interest, with interest
compounded once a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100 + 10% = $110
$110 + 10% = $110 + $11 = $121
. . . . . .
44. Compounded Interest
Question
Suppose you save $100 at 10% annual interest, with interest
compounded once a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100 + 10% = $110
$110 + 10% = $110 + $11 = $121
$100(1.1)t .
. . . . . .
45. Compounded Interest: quarterly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded four times a year. How much do you have
After one year?
After two years?
after t years?
. . . . . .
46. Compounded Interest: quarterly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded four times a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100(1.025)4 = $110.38,
. . . . . .
47. Compounded Interest: quarterly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded four times a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100(1.025)4 = $110.38, not $100(1.1)4 !
. . . . . .
48. Compounded Interest: quarterly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded four times a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100(1.025)4 = $110.38, not $100(1.1)4 !
$100(1.025)8 = $121.84
. . . . . .
49. Compounded Interest: quarterly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded four times a year. How much do you have
After one year?
After two years?
after t years?
Answer
$100(1.025)4 = $110.38, not $100(1.1)4 !
$100(1.025)8 = $121.84
$100(1.025)4t .
. . . . . .
50. Compounded Interest: monthly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded twelve times a year. How much do you have after t
years?
. . . . . .
51. Compounded Interest: monthly
Question
Suppose you save $100 at 10% annual interest, with interest
compounded twelve times a year. How much do you have after t
years?
Answer
$100(1 + 10%/12)12t
. . . . . .
52. Compounded Interest: general
Question
Suppose you save P at interest rate r, with interest compounded n
times a year. How much do you have after t years?
. . . . . .
53. Compounded Interest: general
Question
Suppose you save P at interest rate r, with interest compounded n
times a year. How much do you have after t years?
Answer
( r )nt
B(t) = P 1 +
n
. . . . . .
54. Compounded Interest: continuous
Question
Suppose you save P at interest rate r, with interest compounded
every instant. How much do you have after t years?
. . . . . .
55. Compounded Interest: continuous
Question
Suppose you save P at interest rate r, with interest compounded
every instant. How much do you have after t years?
Answer
( ( )
r )nt 1 rnt
B(t) = lim P 1 + = lim P 1 +
nāā n nāā n
[ ( )n ]rt
1
= P lim 1 +
nāā n
independent of P, r, or t
. . . . . .
63. Existence of e
See Appendix B
( )n
1
n 1+
n
1 2
2 2.25
3 2.37037
10 2.59374
100 2.70481
1000 2.71692
106 2.71828
. . . . . .
64. Existence of e
See Appendix B
( )n
1
We can experimentally n 1+
n
verify that this number
1 2
exists and is
2 2.25
e ā 2.718281828459045 . . . 3 2.37037
10 2.59374
100 2.70481
1000 2.71692
106 2.71828
. . . . . .
65. Existence of e
See Appendix B
( )n
1
We can experimentally n 1+
n
verify that this number
1 2
exists and is
2 2.25
e ā 2.718281828459045 . . . 3 2.37037
10 2.59374
e is irrational 100 2.70481
1000 2.71692
106 2.71828
. . . . . .
66. Existence of e
See Appendix B
( )n
1
We can experimentally n 1+
n
verify that this number
1 2
exists and is
2 2.25
e ā 2.718281828459045 . . . 3 2.37037
10 2.59374
e is irrational 100 2.70481
e is transcendental 1000 2.71692
106 2.71828
. . . . . .
67. Meet the Mathematician: Leonhard Euler
Born in Switzerland,
lived in Prussia
(Germany) and Russia
Eyesight trouble all his
life, blind from 1766
onward
Hundreds of
contributions to
calculus, number theory,
graph theory, ļ¬uid
mechanics, optics, and
astronomy
Leonhard Paul Euler
Swiss, 1707ā1783
. . . . . .
68. A limit
Question
eh ā 1
What is lim ?
hā0 h
. . . . . .
69. A limit
Question
eh ā 1
What is lim ?
hā0 h
Answer
If h is small enough, e ā (1 + h)1/h . So
eh ā 1
ā1
h
. . . . . .
70. A limit
Question
eh ā 1
What is lim ?
hā0 h
Answer
If h is small enough, e ā (1 + h)1/h . So
eh ā 1
ā1
h
eh ā 1
In fact, lim = 1.
hā0 h
This can be used to characterize e:
2h ā 1 3h ā 1
lim = 0.693 Ā· Ā· Ā· < 1 and lim = 1.099 Ā· Ā· Ā· < 1
hā0 h hā0 h
. . . . . .