Applications of Differentiation

Applications
of
Differentiation
JOEY F. VALDRIZ
Mathematical Analysis I

Applications of Differentiation
• Extrema on an Interval
• The Rolle’s Theorem and the
Mean Value Theorem
• Increasing and Decreasing Functions and
the First Derivative Test

0
1
2
3
4
-2 -1 1 2
2
y x
 0,2D 
Absolute
Minimum
Absolute
Maximum


0
1
2
3
4
-2 -1 1 2
2
y x
 0,2D 
No Minimum
Absolute
Maximum


0
1
2
3
4
-2 -1 1 2
2
y x
 0,2D 
No Minimum
No Maximum


8
Extreme Value Theorem
If f is continuous over a closed interval, then f
has a maximum and minimum value over that
interval.
Maximum &
minimum
at interior points
Maximum &
minimum
at endpoints
Maximum at
interior point,
minimum at
endpoint


9
Relative Extrema and Critical Numbers
The graph of f(x) = x3 – 3x2
has a relative maximum at
the point (0, 0) and a
relative minimum at the
point (2, –4).

10

11
Example 1 – The Value of the Derivative at Relative Extrema
Find the value of the derivative at each relative extremum
shown below:

12
Example 1 – The Value of the Derivative at Relative Extrema
Find the value of the derivative at each relative extremum
shown below:

13
Example 1(a) – Solution
At the point (3, 2), the value of the derivative is f'(3) = 0
(see the figure below).

14
Example 1(b) – Solution
At x = 0, the derivative of f(x) = |x| does not exist because
the following one-sided limits differ (see the given figure).
cont’d

15
Example 1(c) – Solution
The derivative of f(x) = sin x is f'(x) = cos x.
At the point (π/2, 1), the value of the
derivative is f '(π/2) = cos(π/2) = 0.
At the point (3π/2, –1), the value of the
derivative is f'(3π/2) = cos(3π/2) = 0
(see the figure)
cont’d

16
Question…
What happens to the derivative
of every function at each
relative extrema?

17

18
Two types of Critical Numbers. Notice in the definition that
the critical number c has to be in the domain of f, but c
does not have to be in the domain of f'.

19
Finding Extrema on a Closed Interval

20
Example 2 – Finding Extrema on a Closed Interval
Find the extrema of f(x) = 3x4 – 4x3 on the interval [–1, 2].
Solution:
Begin by differentiating the function.
f(x) = 3x4 – 4x3 Write original function.
f'(x) = 12x3 – 12x2 Differentiate.

21
Example 2 – Solution
To find the critical numbers of f, you must find all x-values
for which f'(x) = 0 and all x-values for which f'(x) does not
exist.
f'(x) = 12x3 – 12x2 = 0 Set f'(x) equal to 0.
12x2(x – 1) = 0 Factor.
x = 0, 1 Critical numbers
Because f' is defined for all x, you can conclude that these
are the only critical numbers of f.
cont’d

22
By evaluating f at these two critical numbers and at the
endpoints of [–1, 2], you can determine that the maximum
and the minimum.
Example 2 – Solution cont’d

23
By evaluating f at these two critical numbers and at the
endpoints of [–1, 2], you can determine that the maximum
is f(2) = 16 and the minimum is f(1) = –1, as shown in the
table.

24

25
More Examples
1. 𝑓 𝑥 = 2𝑥 − 3𝑥
2
3 [-1, 3]
2. 𝑓 𝑥 = 𝑥
2
3 [-2, 3]
3. 𝑓 𝑥 = 𝑥3
− 3𝑥2
[-1, 3]
4. 𝑓 𝑥 = 4𝑥3 + 5𝑥2 − 42𝑥 + 7
5. 𝑓 𝑥 = (4𝑥 + 1) 𝑥2 − 16

Rolle’s Theorem
 The Extreme Value Theorem states that a continuous
function on a closed interval [a, b] must have both a minimum
and a maximum on the interval.
 Both of these values, however, can occur at the endpoints.
Rolle’s Theorem, named after the French mathematician
Michel Rolle, gives conditions that guarantee the existence of
an extreme value in the interior of a closed interval.

27
Rolle’s Theorem
From Rolle’s Theorem, you can see that if a function f is
continuous on [a, b] and differentiable on (a, b), and if
f(a) = f(b), then there must be at least one x-value between
a and b at which the graph of f has a horizontal tangent, as
shown in the figure below.
f is continuous on [a, b] and differentiable on (a, b).

28
Rolle’s Theorem
If the differentiability requirement is dropped from Rolle’s
Theorem, f will still have a critical number in (a, b), but it
may not yield a horizontal tangent. Such a case is shown in
the figure below.
f is continuous on [a, b].

29
Example 1 – Illustrating Rolle’s Theorem
Find the two x-intercepts of
f(x) = x2 – 3x + 2
and show that f(x) = 0 at some point between the two
x-intercepts.
Solution:
Note that f is differentiable on the entire real number line.
Setting f(x) equal to 0 produces
x2 – 3x + 2 = 0
(x – 1)(x – 2) = 0.
Set f(x) equal to 0.
Factor.

30
So, f(1) = f(2) = 0, and from Rolle’s Theorem you know that
there exists at least one c in the interval (1, 2) such that
f(c) = 0.
To find such a c, you can solve the equation
f(x) = 2x – 3 = 0
and determine that f(x) = 0 when
cont’d
Set f(x) equal to 0.

31
Note that this x-value lies in the open interval (1, 2), as
shown in the given figure.
cont’d
The x-value for which f (x) = 0 is between
the two x-intercepts.

32
More Examples
1. 𝑓 𝑥 =
𝑥2−2𝑥−3
𝑥+2
[-1, 3]
2. 𝑓 𝑥 = sin(2𝑥) [
𝜋
6
,
𝜋
3
]
3. 𝑓 𝑥 = 𝑥2 − 4𝑥 + 1 [0, 4]
4. 𝑓 𝑥 = (𝑥2
− 2𝑥)) 𝑒 𝑥
[0, 2]

The Mean Value Theorem
 Rolle’s Theorem can be used to prove another
theorem—the Mean Value Theorem.

34
Geometrically, the theorem guarantees the existence of a
tangent line that is parallel to the secant line through the
points (a, f(a)) and (b, f(b)), as shown in the figure below.

35
Example 3 – Finding a Tangent Line
Given f(x) = 5 – (4/x), find all values of c in the open
interval (1, 4) such that
Solution:
The slope of the secant line through (1, f(1)) and (4, f(4)) is

36
Note that the function satisfies the conditions of the Mean
Value Theorem. That is, f is continuous on the interval
[1, 4] and differentiable on the interval (1, 4).
So, there exists at least one number c in (1, 4) such that
f(c) = 1. Solving the equation f(x) = 1 yields
which implies that x = 2.
cont’d

37
So, in the interval (1, 4), you can conclude that c = 2, as
shown in the figure below.
cont’d
The tangent line at (2, 3) is parallel to the
secant line through (1, 1) and (4, 4).
Figure 4.13

38
A useful alternative form of the Mean Value Theorem is as
follows: If f is continuous on [a, b] and differentiable on
(a, b), then there exists a number c in (a, b) such that
Alternative form of Mean Value
Theorem

39
More Examples
1. 𝑓 𝑥 = 3𝑥3 − 2𝑥2 − 𝑥 [-1, 2]
2. 𝑓 𝑥 = 𝑥2 − 4𝑥 [2, 4]
3. 𝑓 𝑥 =
1
𝑥
[1, 2]

40
Increasing and
Decreasing Functions

41
A function is increasing if, as x moves to the right, its graph
moves up, and is decreasing if its graph moves down.
The function below is decreasing on the interval is
constant on the interval (a, b) and is increasing on the
interval
Increasing and Decreasing Functions

42

43
Find the open intervals on which is
increasing or decreasing.
Solution:
Note that f is differentiable on the entire real number line.
To determine the critical numbers of f, set f'(x) equal to
zero.
Example 1 – Intervals on Which f Is Increasing or Decreasing

44
Because there are no points for which f' does not exist, you
can conclude that x = 0 and x = 1 are the only critical
numbers.

45
cont’d
numbers.

46
cont’d
numbers.

47
cont’d
numbers.

48
So, f is increasing on the intervals and
and decreasing on the interval (0, 1).
cont’d

49
A function is strictly monotonic on an interval if it is either
increasing on the entire interval or decreasing on the entire
interval. For instance, the function f(x) = x3
is strictly monotonic on the entire real number line because
it is increasing on the entire real number line.

50
The function shown in the figure below is not strictly
monotonic on the entire real number line because it is
constant on the interval [0, 1].

More Examples
1. 𝑓 𝑥 = 𝑥 + 2 2
𝑥 − 1
2. 𝑓 𝑥 = 2𝑥3 + 3𝑥2 − 12𝑥
51

The First Derivative Test
The intervals on which a function is increasing or
decreasing, it is not difficult to locate the relative extrema of
the function.
Example:
A relative maximum at the point
(0, 0) because f is increasing
immediately to the left of x = 0
and decreasing immediately to the
right of x = 0.52

The First Derivative Test
Similarly, f has a relative minimum at the point
because f is decreasing immediately to the left
of x = 1 and
increasing immediately
to the right of x = 1.
53

55
Example 2 – Applying the First Derivative Test
Find the relative extrema of the function
in the interval (0, 2π).
Solution:
Note that f is continuous on the interval (0, 2π).
To determine the critical numbers of f in this interval, set
f'(x) equal to 0.

56
Because there are no points for which f ' does not exist,
you can conclude that x = π/3 and x = 5π/3 are the only
critical numbers.
cont'd

57
critical numbers.
cont'dExample 2 – Solution

58
critical numbers.
cont'dExample 2 – Solution

59
critical numbers.
cont'd

60
By applying the First Derivative Test, you can conclude that
f has a relative minimum at the point where
and a relative maximum at the point
where
cont'd

More Examples
1. 𝑓 𝑥 = 𝑥 + 2 2
𝑥 − 1
2. 𝑓 𝑥 = 2𝑥3 + 3𝑥2 − 12𝑥
61

Applications of Differentiation

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