Sections 4.1 and 4.2 Linear Functions and Their Properties Linear Models.
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Sections 4.1 and 4.2
Linear Functions
and Their Properties
Linear Models
Linear Function
A linear function can be expressed in the form
( )f x mx b
where m and b are fixed numbers.
y mx b Equation notation
Function notation
Graph of a Linear Function
The graph of a linear function is a straight line.
( )y f x mx b
m is called the slope of the line and
b is the y-intercept of the line.
This means that we need only two points to completely determine its graph.
y-axis
x-axis
(1,2)
Example: Sketch the graph of f (x) = 3x – 1
y-intercept
Arbitrary point
(0,-1)
Graph of a Linear Function
The Role of m (slope)
f changes m units for each one-unit change in x.
The Role of b (y-intercept)
when x = 0, f (0) = b
Role of m and b in f (x) = mx + b
To see how f changes, consider a unit change in x.
( )f x mx b ( 1) ( 1)f x m x b
Then, the change in f is given by
( 1) ( )f x f x ( 1)m x b ( )mx b ( 1) ( )f x f x mx m b mx b ( 1) ( )f x f x m
Role of m and b in f (x) = mx + b
Role of m and b in f (x) = mx + b
Role of m and b in f (x) = mx + b
Role of m and b in f (x) = mx + b
y-axis
x-axis
(1,2)
Example: Sketch the graph of f (x) = 3x – 1
y-intercept
Slope = 3/1
Using the Slope and y-Intercept
Graphing a Line Using Intercepts
y-axis
x-axis
Example: Sketch 3x + 2y = 6
y-intercept (x = 0)
x-intercept (y = 0)
Delta Notation
2 1q q q
If a quantity q changes from q1 to q2 , the change in q is denoted by q and it is computed as
Example: If x is changed from 2 to 5, we write
2 1 5 2 3x x x
Delta Notation
Example: the slope of a non-vertical line that passes through the points (x1 , y1) and (x2 , y2) is given by:
2 1
2 1
y yym
x x x
Example: Find the slope of the line that passes through the points (4,0) and (6, -3)
3 0 3 3
6 4 2 2
ym
x
Delta Notation
Example: Find the slope of the line that passes through the points (4,5) and (2, 5).
Example: Find the slope of the line that passes through the points (4,1) and (4, 3).
5 5 00
2 4 2
ym
x
3 1 2
4 4 0
ym
x
Undefined This is a vertical line
This is a horizontal line
Zero Slope and Undefined Slope
Estimate the slope of all line segments in the figure
Examples
Point-Slope Form of the Line
1 1
1 4 3
4 11
y y m x x
y x
y x
An equation of a line that passes through the point (x1 , y1) with slope m is given by:
Example: Find an equation of the line that passes through (3,1) and has slope m = 4
1 1y y m x x
Horizontal Lines
y = 2
Can be expressed in the form y = b
Vertical Lines
x = 3
Can be expressed in the form x = a
Linear Models: Applications of linear
Functions
First,General Definitions
Cost Function
A cost function specifies the cost C as a function of the number of items x produced. Thus, C(x) is the total cost of producing x items.
The cost functions is made up of two parts: C(x)= “variable costs” + “fixed costs”
If the graph of a cost function is a straight line, then we have a Linear Cost Function.
If the graph is not a straight line, then we have a Nonlinear Cost Function.
Cost Function
Linear Cost Function
Dollars
Units
Cost
Dollars
Units
Cost
Dollars
Units
CostDollars
Units
Cost
Non-Linear Cost Function
Revenue Function
The revenue function specifies the total payment received R from selling x items. Thus, R(x) is the revenue from selling x items.
A revenue function may be Linear or Nonlinear depending on the expression that defines it.
Linear Revenue Function
Dollars
Units
Revenue
Nonlinear Revenue Functions
Dollars
Units
RevenueDollars
Units
Revenue
Profit Function The profit function specifies the net proceeds P.
Thus P represents what remains of the revenue when costs are subtracted. Thus, P(x) is the profit from selling x items.
A profit function may be linear or nonlinear depending on the expression that defines it.
Profit = Revenue – Cost
Linear Profit Function
Dollars
Units
Profit
Nonlinear Profit Functions
Dollars
Units
ProfitDollars
Units
Profit
The Linear Models are
Cost Function: 1( )C x m x b
* m1 is the marginal cost (cost per item), b is fixed cost.
Revenue Function: 2( )R x m x* m2 is the marginal revenue.
Profit Function: ( ) ( ) ( )P x R x C x
where x = number of items (produced and sold)
Break-Even Analysis
The break-even point is the level of production that results in no profit and no loss.
To find the break-even point we set the profit function equal to zero and solve for x.
( ) 0
( ) ( ) 0
P x
R x C x
Break-Even Analysis
Profit = 0 means Revenue = Cost
Dollars
Units
loss
Revenue
Cost
profit
Break-even point
Break-even Revenue
The break-even point is the level of production that results in no profit and no loss.
Example: A shirt producer has a fixed monthly cost of $3600. If each shirt has a cost of $3 and sells for $12 find:
a. The cost function
b. The revenue function
c. The profit from 900 shirts
C (x) = 3x + 3600 where x is the number of shirts produced.
R (x) = 12x where x is the number of shirts sold.
P (x) = R(x) – C(x)
P (x) = 12x – (3x + 3600) = 9x – 3600
P(900) = 9(900) – 3600 = $4500
C (x) = R (x)
Example: A shirt producer has a fixed monthly cost of $3600. If each shirt has a cost of $3 and sells for $12 find the break-even point.
The break even point is the solution of the equation
12 3 3600
400
x x
x
and (400) 4800R
Therefore, at 400 units the break-even revenue is $4800
-200 200 400 600 800 1000 1200 1400
-4000
-3000
-2000
-1000
1000
2000
3000
4000
5000
6000
7000
x = number of shirts sold
x
C, R, P in $
C(x)
-200 200 400 600 800 1000 1200 1400
-4000
-3000
-2000
-1000
1000
2000
3000
4000
5000
6000
7000
x = number of shirts sold
x
C, R, P in $
C(x)R(x)
-200 200 400 600 800 1000 1200 1400
-4000
-3000
-2000
-1000
1000
2000
3000
4000
5000
6000
7000
x = number of shirts sold
x
C, R, P in $
C(x)R(x)
P(x)
Demand Function
A demand function or demand equation expresses the number q of items demanded as a function of the unit price p (the price per item).
Thus, q(p) is the number of items demanded when the price of each item is p.
As in the previous cases we have linear and nonlinear demand functions.
Linear Demand Function
q = items demanded
Price p
Nonlinear Demand Functions
q = items demanded
q = items demanded
Price p Price p
Supply Function
A supply function or supply equation expresses the number q of items, a supplier is willing to make available, as a function of the unit price p (the price per item).
Thus, q(p) is the number of items supplied when the price of each item is p.
As in the previous cases we have linear and nonlinear supply functions.
Linear Supply Function
q = items supplied
Price p
Nonlinear Supply Functions
q = items supplied
q = items supplied
Price p Price p
Market Equilibrium
Market Equilibrium occurs when the quantity produced is equal to the quantity demanded.
q
p
supply curve
demand curve
Equilibrium Point
shortage surplus
q
p
shortage
supply curve
demand curve
surplus
Equilibrium price
Equilibrium demand
Market Equilibrium
Market Equilibrium occurs when the quantity produced is equal to the quantity demanded.
To find the Equilibrium price set the demand equation equal to the supply equation and solve for the price p.
To find the Equilibrium demand evaluate the demand (or supply) function at the equilibrium price found in the previous step.
Market Equilibrium
Example of Linear Demand
The quantity demanded of a particular computer game is 5000 games when the unit price is $6. At $10 per unit the quantity demanded drops to 3400 games.
1 1 2 2( , ) (6,50) and ( , ) (10, 34)p q p q
2 1
2 1
34 50 164
10 6 4
q qqm
p p p
4 74q p 50 4( 6)q p
Find a linear demand equation relating the price p, and the quantity demanded, q (in units of 100).
Example: The maker of a plastic container has determined that the demand for its product is 400 units if the unit price is $3 and 900 units if the unit price is $2.50.
The manufacturer will not supply any containers for less than $1 but for each $0.30 increase in unit price above the $1, the manufacturer will market an additional 200 units. Assume that the supply and demand functions are linear. Let p be the price in dollars, q be in units of 100 and find:
a. The demand function
b. The supply function
c. The equilibrium price and equilibrium demand
a. The demand function
1 1 2 2, 3, 4 and , 2.5,9 ;p q p q
9 410
2.5 3
qm
p
4 10 3q p
10 34q p b. The supply function
1 1 2 2, 1,0 and , 1.3,2 ;p q p q
2 0 20
1.3 1 3
qm
p
20 20
3 3q p
Graphical Interpretation
c. The equilibrium price and equilibrium demand
10 34 (1/ 3)(20 20)p p 30 102 20 20
2.44
p p
p
The equilibrium demand is 960 units at a price of $2.44 per unit.
10(2.44) 34 9.6q
Demand Supply
More Linear Models from Verbal Descriptions
Straight-line Depreciation
Let ( ) represent the value of each car after years.V x x(0) $28,000 and the slope is 4000 since the car depreciates
by that amount per year.
V ( ) 4000 28000V x x
3 4000 3 2800 16000V
Straight-line Depreciation
(d) The slope is the average rate of change and is 4000 so this means that for each additional year that passes, the book value of the car decreases by $4000.
(e) 8000 4000 28000x
20000 4000x 20000
54000
x
The car will have a book value of $8000 when it is 5 years old.
Straight-line Depreciation
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