Post on 28-Dec-2015
Modeling and Optimization
Buffalo Bill’s Ranch, North Platte, Nebraska Greg Kelly, Hanford High School, Richland, WashingtonAdapted by: Jon Bannon, Siena College
Photo by Vickie Kelly, 1999
A Classic Problem
You have 40 feet of fence to enclose a rectangular garden along the side of a barn. What is the maximum area that you can enclose?
x x
40 2x
40 2A x x
240 2A x x
40 4A x
0 40 4x
4 40x
10x 40 2l x
w x 10 ftw
20 ftl
There must be a local maximum here, since the endpoints are minimums.
A Classic Problem
You have 40 feet of fence to enclose a rectangular garden along the side of a barn. What is the maximum area that you can enclose?
x x
40 2x
40 2A x x
240 2A x x
40 4A x
0 40 4x
4 40x
10x
10 40 2 10A
10 20A
2200 ftA40 2l x
w x 10 ftw
20 ftl
To find the maximum (or minimum) value of a function:
1 Write it in terms of one variable.
2 Find the first derivative and set it equal to zero.
3 Check the end points if necessary.
Example 5: What dimensions for a one liter cylindrical can will use the least amount of material?
We can minimize the material by minimizing the area.
22 2A r rh area ofends
lateralarea
We need another equation that relates r and h:
2V r h
31 L 1000 cm21000 r h
2
1000h
r
22
10 02
02A r r
r
2 20002A r
r
2
20004A r
r
Example 5: What dimensions for a one liter cylindrical can will use the least amount of material?
22 2A r rh area ofends
lateralarea
2V r h
31 L 1000 cm21000 r h
2
1000h
r
22
10 02
02A r r
r
2 20002A r
r
2
20004A r
r
2
20000 4 r
r
2
20004 r
r
32000 4 r
3500r
3500
r
5.42 cmr
2
1000
5.42h
10.83 cmh
If the end points could be the maximum or minimum, you have to check.
Notes:
If the function that you want to optimize has more than one variable, use substitution to rewrite the function.
If you are not sure that the extreme you’ve found is a maximum or a minimum, you have to check.