MAT1005
Week 2(B)
Chapter 1
Functions and Graphs
Section 1
Functions
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Graphing an Equation
To sketch the graph an equation in x and y, we need to find
ordered pairs that solve the equation and plot the ordered
pairs on a grid. This process is called point-by-point
plotting.
For example, let’s plot the graph of the equation
y x 2
2
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Graphing an Equation:
Making a Table of Ordered Pairs
Make a table of ordered x y
pairs that satisfy the
–3 (–3)2+2 = 11
equation
–2 (–2)2+2 = 6
y x 2
2
–1 (–1)2+2 = 3
0 (0)2+2 = 2
1 (1)2+2 = 3
2 (2)2+2 = 6
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(x, y)
(–3,11)
(–2, 6)
(–1, 3)
(0, 2)
(1, 3)
(2, 6)
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Graphing an Equation:
Plotting the points
Next, plot the points and connect them with a smooth
curve. You may need to plot additional points to see the
pattern formed.
y = x2 - 2
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Functions
The previous graph is the graph of a function. The idea of
a function is this: a correspondence between two sets D
and R such that to each element of the first set, D, there
corresponds one and only one element of the second set,
R.
The first set is called the domain, and the set of
corresponding elements in the second set is called the
range.
For example, the cost of a pizza (C) is related to the size of
the pizza. A 10 inch diameter pizza costs $9.00, while a 16
inch diameter pizza costs $12.00.
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Function Definition
You can visualize a function by the following diagram which
shows a correspondence between two sets: D, the domain of
the function, gives the diameter of pizzas, and R, the range of
the function gives the cost of the pizza.
10 9.00
12 10.00
16
12.00
domain D range R
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Functions Specified by Equations
If in an equation in two variables, we get exactly one
output (value for the dependent variable) for each input
(value for the independent variable), then the equation
specifies a function. The graph of such a function is just
the graph of the specifying equation.
If we get more than one output for a given input, the
equation does not specify a function.
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Functions Specified by Equations
Consider the equation that was graphed on a previous slide
y x 2 2
–2
Input:
x = –2
Process:
(–2,2) is an 2 2
2 square (–2),
then subtract 2
ordered pair of
the function.
Output:
2 result is 2
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Vertical Line Test for a Function
If you have the graph of an equation, there is an easy
way to determine if it is the graph of an function. It is
called the vertical line test which states that:
An equation specifies a function if each vertical line in
the coordinate system passes through at most one
point on the graph of the equation.
If any vertical line passes through two or more points
on the graph of an equation, then the equation does
not specify a function.
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Vertical Line Test for a Function
(continued)
This graph is not the graph of a
function because you can draw a
vertical line which crosses it
twice.
This is the graph of a
function because any vertical
line crosses only once.
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Function Notation
The following notation is used to describe functions. The
variable y will now be called f (x).
This is read as “ f of x” and simply means the y coordinate
of the function corresponding to a given x value.
Our previous equation
y x 2 2
can now be expressed as
f ( x) x 2 2
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Function Evaluation
Consider our function f ( x) x 2 2
What does f (–3) mean?
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Function Evaluation
Consider our function f ( x) x 2 2
What does f (–3) mean?
Replace x with the value –3 and evaluate the expression
f (3) (3)2 2
The result is 7 . This means that the point (–3,7) is on the
graph of the function.
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Some Examples
1.
f (x) 3x 2
f (2) 3(2) 2 4 2
f (a) 3(a) 2
f (6 h) 3(6 h) 2 18 3h 2
16 3h
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Domain of a Function
Consider f ( x) 3 x 2
f (0) ?
f (0) 3(0) 2 2
which is not a real number.
Question: for what values of x is the function defined?
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Domain of a Function
Answer: f ( x) 3 x 2
is defined only when the radicand (3x – 2) is equal to
or greater than zero. This implies that
2
x
3
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Domain of a Function
(continued)
Therefore, the domain of our function is the set of real
numbers that are greater than or equal to 2/3.
Example: Find the domain of the function
1
f ( x) x4
2
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Domain of a Function
(continued)
Therefore, the domain of our function is the set of real
numbers that are greater than or equal to 2/3.
Example: Find the domain of the function
1
f ( x) x4
2
Answer:
x x 8 , [8, )
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Domain of a Function:
Another Example
1
Find the domain of f ( x)
3x 5
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Domain of a Function:
Another Example
1
Find the domain of f ( x)
3x 5
In this case, the function is defined for all values of x
except where the denominator of the fraction is zero. This
means all real numbers x except 5/3.
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Mathematical Modeling
The price-demand function for a company is given by
p(x) 1000 5x, 0 x 100
where x represents the number of items and p(x) represents the
price of the item. Determine the revenue function and find the
revenue generated if 50 items are sold.
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Solution
Revenue = Price ∙ Quantity, so
R(x)= p(x) ∙ x = (1000 – 5x) ∙ x
When 50 items are sold, x = 50, so we will evaluate the
revenue function at x = 50:
R(50) 1000 5 50 50 750 50 37,500
The domain of the function has already been specified. We
are told that
0 x 100
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Break-Even and Profit-Loss
Analysis
Any manufacturing company has costs C and revenues R.
The company will have a loss if R < C, will break even
if R = C, and will have a profit if R > C.
Costs include fixed costs such as plant overhead, etc. and variable
costs, which are dependent on the number of items produced.
C = a + bx
(x is the number of items produced)
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Break-Even and Profit-Loss
Analysis
(continued)
Price-demand functions, usually determined by financial
departments, play an important role in profit-loss analysis.
p = m – nx
(x is the number of items than can be sold at $p per item.)
The revenue function is
R = (number of items sold) ∙ (price per item)
= xp = x(m – nx)
The profit function is
P = R – C = x(m – nx) – (a + bx)
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Example of Profit-Loss Analysis
A company manufactures notebook computers. Its
marketing research department has determined that the
data is modeled by the price-demand function
p(x) = 2,000 – 60x, when 1 < x < 25, (x is in thousands).
What is the company’s revenue function and what is its
domain?
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Answer to Revenue Problem
Since Revenue = Price ∙ Quantity,
R( x) x p( x) x (2000 60 x) 2000 x 60 x 2
The domain of this function is the same as the domain
of the price-demand function, which is 1 ≤ x ≤ 25 (in
thousands.)
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Profit Problem
The financial department for the company in the preceding
problem has established the following cost function for
producing and selling x thousand notebook computers:
C(x) = 4,000 + 500x (x is in thousand dollars).
Write a profit function for producing and selling x thousand
notebook computers, and indicate the domain of this function.
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Answer to Profit Problem
Since Profit = Revenue – Cost, and our revenue function
from the preceding problem was R(x) = 2000x – 60x2,
P(x) = R(x) – C(x) = 2000x – 60x2 – (4000 + 500x)
= –60x2 + 1500x – 4000.
The domain of this function is the same as the domain of
the original price-demand function, 1< x < 25 (in
thousands.) 5000
Thousand dollars
25
Thousand notebook computers
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