Date post: | 24-Dec-2015 |
Category: |
Documents |
Upload: | damon-kelley-harmon |
View: | 238 times |
Download: | 0 times |
Chapter 6The Integral
Sections 6.1, 6.2, and 6.3
The Indefinite Integral
Substitution
The Definite Integral As a Sum
The Definite Integral As Area
The Integral
A physicist who knows the velocity of a particle might wish to know its position at a given time.
A biologist who knows the rate at which a bacteria population is increasing might want to deduce what the size of the population will be at some future time.
Introduction
In each case, the problem is to find a function F whose derivative is a known function f.
If such a function F exists, it is called an antiderivative of f.
Antiderivatives
Definition
A function F is called an antiderivative of f on
an interval I if F’(x) = f (x) for all x in I.
For instance, let f (x) = x2.
• It is not difficult to discover an antiderivative of f if we keep the Power Rule in mind.
• In fact, if F(x) = ⅓ x3, then F’(x) = x2 = f (x).
Antiderivatives
However, the function G(x) = ⅓ x3 + 100 also satisfies G’(x) = x2.
• Therefore, both F and G are antiderivatives of f.
Indeed, any function of the form H(x)=⅓ x3 + C, where C is a constant, is an antiderivative of f.
• The question arises: Are there any others?
Antiderivatives
If F is an antiderivative of f on an interval I, then the most general antiderivative of f on I is
F(x) + C
where C is an arbitrary constant.
Theorem
Antiderivatives
Going back to the function f (x) = x2, we see that the general antiderivative of f is ⅓ x3 + C.
Antiderivatives
Family of Functions By assigning specific values to C, we obtain a
family of functions.
• Their graphs are vertical
translates of one another.
• This makes sense, as each
curve must have the same
slope at any given value
of x.
Notation for Antiderivatives
The symbol is traditionally used to represent the most general an antiderivative of f on an open interval and is called the indefinite integral of f .
Thus, means F’(x) = f (x)
( )f x dx
( ) ( )F x f x dx
( )f x dxThe expression:
read “the indefinite integral of f with respect to x,” means to find the set of all antiderivatives of f.
( )f x dx
Integral sign Integrand
x is called the variable of integration
Indefinite Integral
For example, we can write
• Thus, we can regard an indefinite integral as representing an entire family of functions (one antiderivative for each value of the constant C).
3 32 2because
3 3
x d xx dx C C x
dx
Indefinite Integral
Every antiderivative F of f must be of the form F(x) = G(x) + C, where C is a constant.
Example: 26 3xdx x C
Represents every possible antiderivative of 6x.
Constant of Integration
1
if 11
nn x
x dx C nn
Example:4
3
4
xx dx C
Power Rule for the Indefinite Integral
1 1lnx dx dx x C
x
x xe dx e C
Indefinite Integral of ex and bx
ln
xx b
b dx Cb
Power Rule for the Indefinite Integral
Sum and Difference Rules
f g dx fdx gdx
Example:
2 2x x dx x dx xdx 3 2
3 2
x xC
( ) ( )kf x dx k f x dx ( constant)k
4 43 32 2 2
4 2
x xx dx x dx C C
Constant Multiple Rule
Example:
Example - Different Variable
Find the indefinite integral:
273 2 6ue u du
u
213 7 2 6ue du du u du du
u
323 7ln 6
3ue u u u C
Position, Velocity, and Acceleration Derivative Form
If s = s(t) is the position function of an object at time t, then
Velocity = v = Acceleration = a = ds
dtdv
dt
Integral Form
( ) ( )s t v t dt ( ) ( )v t a t dt
Integration by Substitution
Method of integration related to chain rule. If u is a function of x, then we can use the formula
/
ff dx du
du dx
Example: Consider the integral:
92 33 5x x dx3 2pick +5, then 3 u x du x dx
10
10
uC
9u du 103 5
10
xC
Sub to get Integrate Back Substitute
23
dudx
x
Integration by Substitution
2Let 5 7 then 10
duu x dx
x
Example: Evaluate
3/ 21
10 3/ 2
uC
3/ 225 7
15
xC
25 7x x dx
2 1/ 215 7
10x x dx u du
Pick u, compute du
Sub in
Sub in
Integrate
3ln
dx
x xLet ln then u x xdu dx
3
3ln
dxu du
x x
2
2
uC
2ln
2
xC
Example: Evaluate
3
3 2
t
t
e dt
e 3
3Let +2 then
3t
t
duu e dt
e
3
3
1 1
32
t
t
e dtdu
ue
ln
3
uC
3ln 2
3
teC
Example: Evaluate
Let f be a continuous function on [a, b]. If F is any antiderivative of f defined on [a, b], then the definite integral of f from a to b is defined by
( ) ( ) ( )b
af x dx F b F a
The Definite Integral
( )b
af x dx is read “the integral, from a to b of f (x) dx.”
In the notation ,
f (x) is called the integrand.
a and b are called the limits of integration; a is the lower limit and b is the upper limit.
For now, the symbol dx has no meaning by itself; is all one symbol. The dx simply indicates that the independent variable is x.
( )b
af x dx
Notation
The procedure of calculating an integral is called
integration. The definite integral is a
number. It does not depend on x.
Also note that the variable x is a “dummy variable.”
( )b
af x dx
( ) ( ) ( )b b b
a a af x dx f t dt f r dr
The Definite Integral
Geometric Interpretationof the Definite Integral
The Definite Integral As Area
The Definite Integral As Net Change of Area
If f is a positive function defined for a ≤ x ≤ b,
then the definite integral represents the
area under the curve y = f (x) from a to b
( )b
af x dx
( )b
aA f x dx
Definite Integral As Area
If f is a negative function for a ≤ x ≤ b, then the
area between the curve y = f (x) and the x-axis
from a to b, is the negative of ( ) .b
af x dx
Definite Integral As Area
Area from to ( )b
aa b f x dx
Consider y = f (x) = 0.5x + 6 on the interval [2,6]
whose graph is given below,
Definite Integral As Area
6
2Find ( )
) by using geometry
) by using the definition
of definite integral
f x dx
a
b
Definite Integral As Area
6
2( ) Area of
Trapezoid
f x dx
Consider y = f (x) = 0.5x + 6 on the interval [2,6]
whose graph is given below,
( )b
af x dx Area of R1 – Area of R2 + Area of R3
a b
R1
R2
R3
If f changes sign on the interval a ≤ x ≤ b, then definite integral represents the net area, that is, a difference of areas as indicated below:
Definite Integral as Net Area
a b
R1
R2
R3
If f changes sign on the interval a ≤ x ≤ b, and we need to find the total area between the graph and the x-axis from a to b, then
Total Area
c d
Total Area Area of R1 + Area of R2 + Area of R3
Area of R1 ( )a
cf x dx
Area of R2 ( )d
cf x dx
Area of R3 ( )d
bf x dx
Example: Use geometry to compute the integral
5
1
1x dx
Area = 2
5
1
1 8 2 6x dx
Area = 8
Area Using Geometry
( ) 1y f x x
–1
5
Example: Use an antiderivative to compute the integral
5
1
1x dx
Area Using Antiderivatives
First, we need an antiderivative of ( ) 1y f x x
21( ) 1 . Thus,
2F x x dx x x C
5
1
15 3
2 21 (5) ( 1) 6C Cx dx F F
Example: Now find the total area bounded by the curve and the x-axis from x –1 to x 5.
Area Using Antiderivatives
( ) 1y f x x
( ) 1y f x x
–11 5
R1
R2
Total Area Area of R1 + Area of R2
( ) 1y f x x
–11 5
R1
R2
Area of R1
121
11
( ) 22
xf x dx x
Area of R2
525
11
( ) 82
xf x dx x
Total Area 2 + 8 10
Evaluating the Definite Integral
Example: Calculate5
1
12 1x dx
x
55 2
1 1
12 1 lnx dx x x x
x
2 25 ln 5 5 1 ln1 1
28 ln 5 26.39056
Substitution for Definite Integrals
1 1/ 22
02 3x x dx
2let 3u x x
then 2
dudx
x
1 41/ 22 1/ 2
0 02 3x x x dx u du
43/ 2
0
2
3u
16
3
Notice limits change
Example: Calculate
Computing Area Example: Find the area enclosed by the x-axis, the vertical lines x = 0, x = 2 and the graph of
23
02x dx Gives the area since 2x3 is
nonnegative on [0, 2].
22
3 4
00
12
2x dx x 4 41 1
2 02 2
8
Antiderivative
22 .y x
The Definite Integral As a Total
If r (x) is the rate of change of a quantity Q (in units of Q per unit of x), then the total or accumulated change of the quantity as x changes from a to b is given by
Total change in quantity ( )b
a
Q r x dx
Example: If at time t minutes you are traveling at a rate of v(t) feet per minute, then the total distance traveled in feet from minute 2 to minute 10 is given by
10
2
Total change in distance ( )v t dt
The Definite Integral As a Total