Transcript Section 4.7
APPLICATIONS OF DIFFERENTIATION
4.7
Optimization Problems
In this section, we will learn:
How to solve problems involving
maximization and minimization of factors.
OPTIMIZATION PROBLEMS
In this section (and the next), we solve
such problems as:
Maximizing areas, volumes, and profits
Minimizing distances, times, and costs
OPTIMIZATION PROBLEMS
Example 1
A farmer has 2400 ft of fencing and wants
to fence off a rectangular field that borders
a straight river. He needs no fence along
the river.
What are the dimensions of the field that
has the largest area?
OPTIMIZATION PROBLEMS
Here are three
possible ways of
laying out the 2400 ft
of fencing.
Example 1
OPTIMIZATION PROBLEMS
Example 1
This figure
illustrates
the general case.
We wish to maximize the area A of
the rectangle.
Let x and y be the depth and width of the rectangle
(in feet).
Then, we express A in terms of x and y: A = xy
OPTIMIZATION PROBLEMS
Example 1
We want to express A as a function of
just one variable.
So, we eliminate y by expressing it in terms of x.
To do this, we use the given information that
the total length of the fencing is 2400 ft.
Thus,
2x + y = 2400
OPTIMIZATION PROBLEMS
Example 1
From that equation, we have:
y = 2400 – 2x
This gives:
A = x(2400 – 2x) = 2400x - 2x2
Note that x ≥ 0 and x ≤ 1200 (otherwise A < 0).
OPTIMIZATION PROBLEMS
Example 1
So, the function that we wish to maximize
is: A(x) = 2400x – 2x2
0 ≤ x ≤ 1200
The derivative is: A’(x) = 2400 – 4x
So, to find the critical numbers, we solve: 2400 – 4x = 0
This gives: x = 600
OPTIMIZATION PROBLEMS
Example 1
The maximum value of A must
occur either at that critical number or
at an endpoint of the interval.
A(0) = 0; A(600) = 720,000; and A(1200) = 0
So, the Closed Interval Method gives the maximum
value as:
A(600) = 720,000
OPTIMIZATION PROBLEMS
Example 1
Thus, the rectangular field should
be:
600 ft deep
1200 ft wide
OPTIMIZATION PROBLEMS
Example 2
A cylindrical can is to be made to
hold 1 L of oil.
Find the dimensions that will minimize
the cost of the metal to manufacture the can.
OPTIMIZATION PROBLEMS
Draw the diagram as in
this figure, where
r is the radius and h the
height (both in
centimeters).
Example 2
OPTIMIZATION PROBLEMS
To minimize the cost of
the metal, we minimize
the total surface area of
the cylinder (top, bottom,
and sides.)
Example 2
OPTIMIZATION PROBLEMS
So, the surface area is:
A = 2πr2 + 2πrh
Example 2
OPTIMIZATION PROBLEMS
Example 2
To eliminate h, we use the fact that
the volume is given as 1 L, which we take
to be 1000 cm3.
Thus,
πr2h = 1000
This gives
h = 1000/(πr2)
OPTIMIZATION PROBLEMS
Example 2
Substituting this in the expression for A gives:
2000
1000
2
A 2 r 2 r
2 r
2
r
r
2
So, the function that we want to minimize is:
2000
A(r ) 2 r
r
2
r 0
OPTIMIZATION PROBLEMS
Example 2
To find the critical numbers, we differentiate:
2000 4( r 500)
A '(r ) 4 r 2
2
r
r
3
3
Then, A’(r) = 0 when πr = 500
So, the only critical number is: r 3 500 /
OPTIMIZATION PROBLEMS
Example 2
2000 4( r 3 500)
A '(r ) 4 r 2
2
r
r
We can observe that A’(r) < 0 for r 3 500 /
and A’(r) > 0 for r 3 500 /
So, A is decreasing for all r to the left of the critical
number and increasing for all r to the right.
Thus, r
minimum.
3
500 / must give rise to an absolute
OPTIMIZATION PROBLEMS
Example 2
The value of h corresponding to
r 500 / is:
3
1000
1000
h
2
r
(500 / ) 2 3
500
3
1000
2
500
500
3
3
500
1000 3
500
23
2r
500
OPTIMIZATION PROBLEMS
Example 2
Thus, to minimize the cost of
the can,
The radius should be r 3 500 / cm
The height should be equal to twice the radius—
namely, the diameter
OPTIMIZATION PROBLEMS
Example 3
Find the point on the parabola
y2 = 2x
that is closest to the point (1, 4).
OPTIMIZATION PROBLEMS
Example 3
The distance between
the point (1, 4) and
the point (x, y) is:
d ( x 1) ( y 4)
However, if (x, y) lies on
the parabola, then x = ½ y2.
So, the expression for d
becomes:
d ( 12 y 2 1)2 ( y 4)2
2
2
OPTIMIZATION PROBLEMS
Example 3
Instead of minimizing d, we minimize
its square:
d f ( y) y 1 y 4
2
1
2
2
2
2
You should convince yourself that the minimum of d
occurs at the same point as the minimum of d2.
However, d2 is easier to work with.
OPTIMIZATION PROBLEMS
Example 3
Differentiating, we obtain:
f '( y ) 2 y 1 y 2( y 4) y 8
1
2
2
So, f’(y) = 0 when y = 2.
3
OPTIMIZATION PROBLEMS
Example 3
Observe that f’(y) < 0 when y < 2 and f’(y) > 0
when y > 2.
So, by the First Derivative Test for Absolute
Extreme Values, the absolute minimum
occurs when y = 2.
f '( y) y 8
3
OPTIMIZATION PROBLEMS
The corresponding value of x is:
x = ½ y2 = 2
Thus, the point on y2 = 2x
closest to (1, 4) is (2, 2).
Example 3
OPTIMIZATION PROBLEMS
A man launches his boat
from point A on a bank of
a straight river, 3 km wide,
and wants to reach point B
(8 km downstream on
the opposite bank) as
quickly as possible.
Example 4
OPTIMIZATION PROBLEMS
He could proceed in any
of three ways:
Row his boat directly across
the river to point C and then run
to B
Row directly to B
Row to some point D between
C and B and then run to B
Example 4
OPTIMIZATION PROBLEMS
If he can row 6 km/h and
run 8 km/h, where should he
land to reach B as soon as
possible?
We assume that the speed of
the water is negligible compared
with the speed at which he rows.
Example 4
OPTIMIZATION PROBLEMS
Example 4
If we let x be the distance from C to D,
then:
The running distance is:
|DB| = 8 – x
The Pythagorean Theorem gives
the rowing distance as:
|AD| = x2 9
OPTIMIZATION PROBLEMS
Example 4
distance
We use the equation time=
rate
Then, the rowing time is: x 2 9 / 6
The running time is: (8 – x)/8
So, the total time T as a function
of x is:
T ( x)
x2 9 8 x
6
8
OPTIMIZATION PROBLEMS
Example 4
The domain of this function T is [0, 8].
Notice that if x = 0, he rows to C, and if x = 8,
he rows directly to B.
x
1
The derivative of T is: T '( x)
6 x2 9 8
OPTIMIZATION PROBLEMS
Example 4
Thus, using the fact that x ≥ 0,
x
1
we have: T '( x) 0
2
6 x 9 8
4x 3 x2 9
16 x 9( x 9)
2
2
9
7 x 81 x
7
2
The only critical number is: 9 / 7 ≈ 3.4
OPTIMIZATION PROBLEMS
Example 4
To see whether the minimum occurs at
this critical number or at an endpoint of
the domain [0, 8], we evaluate T at all three
points:
T (0) 1.5
7
9
T
1 8 1.33
7
73
T (8)
1.42
6
OPTIMIZATION PROBLEMS
Since the smallest of
these values of T
occurs when x = 9 /
7,
the absolute minimum
value of T must occur
there.
The figure illustrates
this calculation by
showing the graph
of T.
Example 4
OPTIMIZATION PROBLEMS
Thus, the man should land the
boat at a point
9 / 7 (≈ 3.4 km)
downstream from his starting
point.
Example 4