1. Calculus I (MAT 145) Dr. Day Wednesday Nov 8, 2017
Using Derivatives: Function Characteristics & Applications (Ch 4)
Solving Problems Involving Optimization
Wednesday, November 8, 2017
MAT 145

2. Optimization What does it mean to optimize?
What are examples in which you might want to determine an optimum solution?
What should you consider when looking for an optimum solution?
Wednesday, November 8, 2017
MAT 145

3. Solve the follow optimization problem
Solve the follow optimization problem. Show complete evidence and calculus justification. Include a drawing or a graph to represent the situation. Include evidence that shows you have considered any domain restrictions.
____ 1 pt: labeled sketch, drawing, graph;
____ 1 pt: variables identified/described;
____ 1 pt: domain of independent variable;
____ 1 pt: statement of optimizing function;
____ 1 pt: constraint;
____ 1 pt: calculus evidence;
____ 1 pt: justify optimum;
____ 1 pt: consider all possibilities for critical points;
____ 1 pt: correct solution, units labeled
Wednesday, November 8, 2017
MAT 145

4. Optimization: An example
A farmer has 2400 ft of fencing and wants to fence off a rectangular field that borders a straight river. She needs no fence along the river. What are the dimensions of the field that has the largest area?
What are we trying to optimize here and in what way (greatest or least)?
Which variables are involved?
What constraints should be taken into account?
What initial guesses do you have about a solution?
Wednesday, November 8, 2017
MAT 145

5. Optimization: Guess & check
A farmer has 2400 ft of fencing and wants to fence off a rectangular field that borders a straight river. She needs no fence along the river. What are the dimensions of the field that has the largest area?
What are options? Guess and check to fit constraints. Are these all the options?
Wednesday, November 8, 2017
MAT 145

6. Optimization: Calculus strategy
A farmer has 2400 ft of fencing and wants to fence off a rectangular field that borders a straight river. She needs no fence along the river. What are the dimensions of the field that has the largest area?
Let x and y be the length and width of the rectangular field (in feet). Then we express A, area of rectangular field (in sq. feet) in terms of x and y: A = xy
Optimization Equation: A=xy
Constraint: 2x + y = 2400
Combine: A = x(2400 – 2x) = 2400x – 2x2
Domain: x is within [0, 1200]
Now, find absolute maximum of A.
(Just like we did when graphing.)
Wednesday, November 8, 2017
MAT 145

7. Optimize
Equation to maximize: A (x) = 2400x – 2x2 , A represents area of field in sq. ft; x represents width of field, as shown in the diagram.
Domain: 0  x  1200; Thus, endpoints to check are x=0, x=1200.
Find derivative and critical numbers (locations at which derivative is zero or undefined):
A (x) = 2400 – 4x
2400 – 4x = 0
x = 600
The absolute maximum value of A must occur either at this critical number or at an endpoint of the interval.
Since A(0) = 0, A(600) = 720,000, and A(1200) = 0, the Closed Interval Method gives the maximum value as A(600) = 720,000.
Dimensions of field with greatest area: 600 ft by 1200 ft.
Note: A’ function is a polynomial; so A’ is defined everywhere on its domain.
Wednesday, November 8, 2017
MAT 145

8. Steps for Optimizing
Understand the problem: What is given? What is requested? Are there constraints?
Diagram and variables: If appropriate for the context draw a diagram and label with variables. Define variables (by saying what the letters you are using represent, including units) and determine a reasonable span of values for those variables in the context of the problem. This will help you identify the domain of the independent variable.
Determine quantity to be optimized and write an equation: Read the problem to determine what you have been asked to optimize. Express the quantity to be optimized (one of your variables), as a function of the other variable(s) in the problem.
Identify constraint equation, if appropriate: If there are several variables, one may be constrained to be a function of the others. Write an equation that relates those variables. Combine with other equation previously defined, if appropriate.
Use the closed interval method to identify the absolute maximum or minimum
Answer the question!
Wednesday, November 8, 2017
MAT 145

9. Open top box
A square piece of cardboard 1 foot on each side is to be cut and folded into a box with no top. To accomplish this, congruent squares will be cut from each corner of the cardboard. Determine the dimensions of the box that will enclose the greatest volume.
Wednesday, November 8, 2017
MAT 145

10. Bendable Wire
A bendable wire measures 1000 cm in length. Write a function, call it A(x), to represent the sum of the areas of a circle and a square that result when that wire is cut at one point and the resulting two pieces of wire are used to create those shapes. Determine how to cut the wire so that the sum of the areas of the circle and the wire are a maximum.
Wednesday, November 8, 2017
MAT 145

11. Bendable Wire
Wednesday, November 8, 2017
MAT 145

12. Bendable Wire
Note: The A’ function is a polynomial; so A’ is defined everywhere on its domain. Thus, the only critical numbers occur when A’ = 0.
A(4000/(π+4)) ≈35,006, A(0)≈79,577, A(1000)=62,500. Thus, max area occurs when x=0 and entire wire is used to make a circle.
Wednesday, November 8, 2017
MAT 145