Indefinite Integrals and the Net Change Theorem

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Sharp Tutor Wed, Jan 19, 2022 06:46 PM UTC

In this section, we:
1. Introduce a notation for antiderivatives.
2. Review the formulas for antiderivatives.
3. Use the formulas to evaluate definite integrals.
4. Reformulate the second part of the FTC (FTC2) in a way that makes it easier to apply to science and engineering problems.

1. 5

2. In Section 5.3, we saw that the second part of
the Fundamental Theorem of Calculus (FTC)
provides a very powerful method for
evaluating the definite integral of a function.
 This is assuming that we can find an antiderivative
of the function.

3. 5.4
Indefinite Integrals and
the Net Change Theorem
In this section, we will learn about:
Indefinite integrals and their applications.

4. INDEFINITE INTEGRALS AND NET CHANGE THEOREM
In this section, we:
 Introduce a notation for antiderivatives.
 Review the formulas for antiderivatives.
 Use the formulas to evaluate definite integrals.
 Reformulate the second part of the FTC (FTC2)
in a way that makes it easier to apply to science
and engineering problems.

5. INDEFINITE INTEGRALS
Both parts of the FTC establish
connections between antiderivatives
and definite integrals.
x
 Part 1 says that if, f is continuous, then
is an antiderivative of f.
 f (t ) dt
a
b
 Part 2 says that a f ( x) dx can be found by evaluating

F(b) – F(a), where F is an antiderivative of f.

6. INDEFINITE INTEGRALS
We need a convenient notation for
antiderivatives that makes them easy
to work with.

7. INDEFINITE INTEGRAL
Due to the relation given by the FTC between
antiderivatives and integrals, the notation
∫ f(x) dx is traditionally used for an
antiderivative of f and is called an indefinite
Thus, ∫ f(x) dx = F(x) means F’(x) = f(x)

8. INDEFINITE INTEGRALS
For example, we can write
3 3
2 x d x  2
x dx  3  C because dx  3  C   x
 Thus, we can regard an indefinite integral
as representing an entire family of functions
(one antiderivative for each value of the constant C).

9. INDEFINITE VS. DEFINITE INTEGRALS
You should distinguish carefully between
definite and indefinite integrals.
b
 A definite integral  f ( x) dx is a number.
a
 An indefinite integral ∫ f(x) dx is a function
(or family of functions).

10. INDEFINITE VS. DEFINITE INTEGRALS
The connection between them is given
by the FTC2.
If f is continuous on [a, b], then
b b
 f ( x ) dx  f ( x ) dx 

a a

11. INDEFINITE INTEGRALS
The effectiveness of the FTC depends
on having a supply of antiderivatives
of functions.
 Therefore, we restate the Table of Antidifferentiation
Formulas from Section 4.9, together with a few others,
in the notation of indefinite integrals.

12. INDEFINITE INTEGRALS
Any formula can be verified by differentiating
the function on the right side and obtaining
the integrand.
2
For instance, sec x dx  tan x  C
because
d
(tan x  C ) sec 2 x
dx

13. TABLE OF INDEFINITE INTEGRALS Table 1
cf ( x) dx c f ( x) dx [ f ( x)  g ( x)] dx f ( x) dx  g ( x) dx
k dx kx  C
n 1
n x 1
 dx  n  1  C (n  1)
x x dx ln | x | C
x x x ax
e dx e C a dx  ln a  C

14. TABLE OF INDEFINITE INTEGRALS Table 1
sin x dx  cos x  C cos x dx sin x  C
2 2
sec x dx tan x  C csc x dx  cot x  C
sec x tan x dx sec x  C csc x cot x dx  csc x  C
1 1 1
x 2  1 dx  tan x C  1 dx sin  1 x  C
x2
sinh x dx cosh x  C cosh x dx sinh x  C

15. INDEFINITE INTEGRALS
Recall from Theorem 1 in Section 4.9 that
the most general antiderivative on a given
interval is obtained by adding a constant to
a particular antiderivative.
 We adopt the convention that, when a formula for
a general indefinite integral is given, it is valid only
on an interval.

16. INDEFINITE INTEGRALS
Thus, we write 1 1
x 2 dx   C
x
with the understanding that it is valid on
the interval (0, ∞) or on the interval (-∞, 0).

17. INDEFINITE INTEGRALS
This is true despite the fact that the general
antiderivative of the function f(x) = 1/x2,
x ≠ 0, is:
 1
   C1 if x  0
x
F ( x) 
 1
 C2 if x  0
 x

18. INDEFINITE INTEGRALS Example 1
Find the general indefinite integral
∫ (10x4 – 2 sec2x) dx
 Using our convention and Table 1, we have:
∫(10x4 – 2 sec2x) dx = 10 ∫ x4 dx – 2 ∫ sec2x dx
= 10(x5/5) – 2 tan x + C
= 2x5 – 2 tan x + C
 You should check this answer by differentiating it.

19. INDEFINITE INTEGRALS Example 2
cos 
Evaluate  2 d
sin 
 This indefinite integral isn’t immediately apparent
in Table 1.
 So, we use trigonometric identities to rewrite
the function before integrating:
cos   1   cos  
sin 2  d  sin    sin   d

 csc  cot  d  csc   C

20. INDEFINITE INTEGRALS Example 3
3
3
Evaluate ( x
0
 6 x) dx
 Using FTC2 and Table 1, we have:
4 2 3
3
3 x x 
0 ( x  6 x) dx  4  6 2 
0
 3  3 3  
1
4
4 2
 1
4
4
0  3 0 2

 814  27  0  0  6.75
 Compare this with Example 2 b in Section 5.2

21. INDEFINITE INTEGRALS Example 4
2  3 3 
0 
 2 x  6 x  2  dx
x 1 
and interpret the result in terms of areas.

22. INDEFINITE INTEGRALS Example 4
The FTC gives:
4 2
2  3 3  x x 1 2
0  2 x  6 x  x 2  1  dx 2 4  6 2  3 tan x  0
4 2 1 2
 x  3x  3 tan x 
1
2 0
4 2 1
 (2 )  3(2 )  3 tan 2  0
1
2
 4  3 tan  1 2
 This is the exact value of the integral.

23. INDEFINITE INTEGRALS Example 4
If a decimal approximation is desired, we can
use a calculator to approximate tan-1 2.
Doing so, we get:
2 3 3 
0 
 2 x  6 x  2  dx  0.67855
x 1 

24. INDEFINITE INTEGRALS
The figure shows the graph of the integrand
in the example.
 We know from Section 5.2
that the value of the
integral can be
interpreted as the sum
of the areas labeled
with a plus sign minus
the area labeled with
a minus sign.

25. INDEFINITE INTEGRALS Example 5
2 2
Evaluate 9 2t  t t  1
1 2
dt
t
 First, we need to write the integrand in a simpler
form by carrying out the division:
9 2t 2  t 2 t  1 9
12 2
1 2
dt  (2  t  t ) dt
t 1

26. INDEFINITE INTEGRALS Example 5
9
12 2
 Then,
 (2  t
1
 t )dt
9
32 1
t t 
2t   
3
2  1 1
9
2 3 2 1
2t  t  
3 t 1
32 32
(2 9  9 2
3  )  (2 1  1  11 )
1
9
2
3
18  18  19  2  2
3  1 32 94

27. The FTC2 says that, if f is continuous on
[a, b], then b
 f ( x) dx  F (b)  F (a)
a
where F is any antiderivative of f.
 This means that F’ = f.
 So, the equation can be rewritten as:
b
 F '( x) dx  F (b)  F (a)
a

28. We know F’(x) represents the rate of change
of y = F(x) with respect to x and F(b) – F(a) is
the change in y when x changes from a to b.
 Note that y could, for instance, increase,
then decrease, then increase again.
 Although y might change in both directions,
F(b) – F(a) represents the net change in y.

29. NET CHANGE THEOREM
So, we can reformulate the FTC2 in words,
as follows.
The integral of a rate of change is
the net change: b
 F '( x) dx  F (b)  F (a)
a

30. NET CHANGE THEOREM
This principle can be applied to all the rates
of change in the natural and social sciences
that we discussed in Section 3.7
The following are a few instances of the idea.

31. NET CHANGE THEOREM
If V(t) is the volume of water in a reservoir at
time t, its derivative V’(t) is the rate at which
water flows into the reservoir at time t.
t2
 So,
 V '(t ) dt V (t
t1 2 )  V (t1 )
is the change in the amount of water
in the reservoir between time t1 and time t2.

32. NET CHANGE THEOREM
If [C](t) is the concentration of the product of
a chemical reaction at time t, then the rate of
reaction is the derivative d[C]/dt.
t2d [C ]
 So,
t1 dt dt [C ](t2 )  [C ](t1 )
is the change in the concentration of C
from time t1 to time t2.

33. NET CHANGE THEOREM
If the mass of a rod measured from the left
end to a point x is m(x), then the linear density
is ρ(x) = m’(x).
b
 So,
  ( x) dx m(b)  m(a)
a
is the mass of the segment of the rod
that lies between x = a and x = b.

34. NET CHANGE THEOREM
If the rate of growth of a population is dn/dt,
t2 dn
 dt n(t2 )  n(t1 )
t1 dt
is the net change in population during the time
period from t1 to t2.
 The population increases when births happen
and decreases when deaths occur.
 The net change takes into account both births
and deaths.

35. NET CHANGE THEOREM
If C(x) is the cost of producing x units of
a commodity, then the marginal cost is
the derivative C’(x).
x2
 So,
 C '( x) dx C ( x )  C ( x )
x1 2 1
is the increase in cost when production
is increased from x1 units to x2 units.

36. NET CHANGE THEOREM Equation 2
If an object moves along a straight line
with position function s(t), then its velocity
is v(t) = s’(t).
t2
 So,
 v(t ) dt s(t
t1 2 )  s (t1 )
is the net change of position, or displacement,
of the particle during the time period from t1 to t2.

37. NET CHANGE THEOREM
In Section 5.1, we guessed that this was
true for the case where the object moves in
the positive direction.
Now, however, we have proved that it is
always true.

38. NET CHANGE THEOREM
If we want to calculate the distance the object
travels during that time interval, we have to
consider the intervals when:
 v(t) ≥ 0 (the particle moves to the right)
 v(t) ≤ 0 (the particle moves to the left)

39. NET CHANGE THEOREM Equation 3
In both cases, the distance is computed by
integrating |v(t)|, the speed.
t2
 | v(t ) | dt  total distance traveled
t1

40. NET CHANGE THEOREM
The figure shows how both displacement and
distance traveled can be interpreted in terms
of areas under a velocity curve.

41. NET CHANGE THEOREM
The acceleration of the object is
a(t) = v’(t).
t2
 So,
 a(t ) dt v(t
t1 2 )  v(t1 )
is the change in velocity from time t1 to time t2.

42. NET CHANGE THEOREM Example 6
A particle moves along a line so that its
velocity at time t is:
v(t) = t2 – t – 6 (in meters per second)
a) Find the displacement of the particle during
the time period 1 ≤ t ≤ 4.
b) Find the distance traveled during this time period.

43. NET CHANGE THEOREM Example 6 a
By Equation 2, the displacement is:
4 4
2
s (4)  s (1)   v(t ) dt   (t  t  6) dt
1 1
3 2 4
t t  9
   6t  
3 2 1 2
 This means that the particle moved 4.5 m
toward the left.

44. NET CHANGE THEOREM Example 6 b
Note that
v(t) = t2 – t – 6 = (t – 3)(t + 2)
 Thus,
v(t) ≤ 0 on the interval [1, 3] and v(t) ≥ 0 on [3, 4]

45. NET CHANGE THEOREM Example 6 b
So, from Equation 3, the distance traveled is:
4 3 4
 v(t ) dt [ v(t )] dt   v(t ) dt
1 1 3
3 4
2 2
( t  t  6) dt   (t  t  6) dt
1 3
3 2 3 3 2 4
 t t  t t 
    6t      6t 
 3 2 1  3 2 3
61
 10.17 m
6

46. NET CHANGE THEOREM Example 7
The figure shows the power consumption in
San Francisco for a day in September.
 P is measured in megawatts.
 t is measured in hours starting at midnight.
Estimate the
energy used
on that day.

47. NET CHANGE THEOREM Example 7
Power is the rate of change of energy:
P(t) = E’(t)
 So, by the Net Change Theorem,
24 24
 P(t ) dt  E '(t ) dt E (24)  E (0)
0 0
is the total amount of energy used that day.

48. NET CHANGE THEOREM Example 7
We approximate the value of
the integral using the Midpoint Rule
with 12 subintervals and ∆t = 2,
as follows.

49. NET CHANGE THEOREM Example 7
24
 P(t ) dt
0
[ P (1)  P(3)  P(5)  ...  P(21)  P(23)]t
(440  400  420  620  790  840  850
 840  810  690  670  550)(2)
 The energy used was approximately
15,840 megawatt-hours.

50. NET CHANGE THEOREM
How did we know what
units to use for energy in
the example?

51. NET CHANGE THEOREM
24
The integral  P(t ) dt is defined as the limit
0
of sums of terms of the form P(ti*) ∆t.
Now, P(ti*) is measured in megawatts and
∆t is measured in hours.
 So, their product is measured in megawatt-hours.
 The same is true of the limit.

52. NET CHANGE THEOREM
In general, the unit of measurement for
b
 f ( x) dx
a
is the product of the unit for f(x) and
the unit for x.

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