PreCalculus
Chapter 1 Section 7
MODELING WITH FUNCTIONS
Functions from Formulas
Write the area A of a circle as a function of its
A. Radius: r
B. Diameter: d
C. Circumference: C
Functions from Formulas
Write the area A of a circle as a function of its
A. Radius:
A(r) = π r2
B. Diameter:
A(d) = π (d/2)2 = (π / 4)d2
Remember: r = d/2
C. Circumference:
A(C ) = π(C/2π)2 = C2/(4π)
Remember: C = 2 π r
Max Volume Problem
A square of side x is cut out of each corner of an 8 in. by 15 in. piece of
cardboard and the sides are folded up to form an open - topped box. See figure
1.80 on page 152.
A. Write the volume V of the box as a function of x.
B. Find the domain of V(x).
Note the model imposes restrictions on x.
C. Graph V(x) over the domain found in part (B) and use the maximum finder
on your grapher to determine the maximum volume of the box.
D. How big should the cut-out squares be in order to produce the box of
maximum volume.
Max Volume Problem
A. The box will have a base with sides of width 8 – 2x and length 15
– 2x. Why?
The depth of the box will be x when the sides are folded up.
Why?
Therefore, V(x) = x(8 – 2x)(15 – 2x)
Max Volume Problem
B. The formula for V is a polynomial with domain of all reals.
However, we are restricted by the reality of the problem. We
cannot cut a negative length. Also, we are restricted by how far
into the cardboard we can cut. What side of the cardboard is
smallest? And how far in can we cut?
We get a relevant domain of [0, 4]
Max Volume Problem
C. Look at the graph in the restricted domain of [0, 4]. Use the
maximum finder on the calculator to determine the maximum
occurs at the point (1.66…, 90.74) or (5/3, 90.47)
D. Each square should have sides of 5/3
Functions from Verbal Descriptions
In this method, finding the function is sometimes harder than
solving the problem.
Be very careful when you read the problem and make sure you
understand it.
Note: many times the rate of change in the problem is represented in
different ways.
◦ For example: the problem may describe the rotation of a tire in
inches per second but may want the answer in miles per hour.
Finding a Model
Grain is leaking through a hole in a storage bin at a constant rate of
8 cubic inches per minute. The grain forms a cone-shaped pile on
the ground below. As it grows, the height of cone always remains
equal to its radius. If the cone is one foot tall now, how tall will it be
in one hour?
What formula did you need to look up in order to solve this problem?
What information do we know that can be used in that formula?
Finding a Model
Vcone = (1/3)π r2 h : However, we are told from the problem that the height is
equal to the radius, and that we want to find the height after an hour.
So our function of Volume with respect to height is: Vcone = (1/3) π h3
When h = 12 inches, V = (1/3) π (12)3 =576π in3
An hour later, the volume will have grown by (60min)(8in3/min) = 480 in3
So, V = 576π +480 = (1/3)π h3 , solve for h.
h3
3(576  480)

 12.98inches
Functions From Data
Given a set of data points of the form (x, y), to construct a formula
that approximates y as a function of x:
1. Make a scatter plot of the data points. The points do not need
to pass the vertical line test.
2. Determine from the shape of the plot whether the points seem
to follow the graph of a familiar type of function (line,
parabola, cubic, sine, etc)
3. Transform a basic function of that type to fit the points as
closely as possible. Do a regression analysis with a graphing
calculator.
Finding Functions from Data
See Example 6 on page 156 for finding a regression line (line of
best fit)
Modeling with Functions
We will continue this idea through Chapter 4.
Please note page 157 in your text.
◦ This page has the basic functions you will need to know and their
applications to “real world” problems.
Homework
# 3 – 39 by multiples of 3 on page 160 & 161.