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Slide1 l.jpg

This is a PowerPoint presentation on fundamental math

tools that are useful in principles of economics.

A left mouse click or the enter key will add an

element to a slide or move you to the next slide. The

backspace key will take you back one element or slide.

The escape key will get you out of

the presentation.

Principles of Microeconomics

ã R. Larry Reynolds


Math review l.jpg
Math Review

  • Mathematics is a very precise language that is useful to express the relationships between related variables

  • Economics is the study of the relationships between resources and the alternative outputs

  • Therefore, math is a useful tool to express economic relationships

Principles of Microeconomics


Relationships l.jpg
Relationships

  • A relationship between two or more variables can be expressed as an equation, table or graph

    • equations & graphs are “continuous”

    • tables contain “discrete” information

      • tables are less complete than equations

      • it is more difficult to see patterns in tabular data than it is with a graph -- economists prefer equations and graphs

Principles of Microeconomics


Equations l.jpg
Equations

  • a relationship between two variables can be expressed as an equation

  • the value of the “dependent variable” is determined by the equation and the value of the “independent variable.”

  • the value of the independent variable is determined outside the equation, i.e. it is “exogenous”

Principles of Microeconomics


Equations cont l.jpg
Equations [cont . . .]

  • An equation is a statement about a relationship between two or more variables

  • Y = fi (X) says the value of Y is determined by the value of X ; Y is a “function of X.”

  • Y is the dependent variable

  • X is the independent variable

  • A linear relationship may be specified: Y = a±mX [the function will graph as a straight line]

    • When X = 0, then Y is “a”

    • for every 1 unit change in X, Y changes by “± m”

Principles of Microeconomics


Y 6 2x l.jpg
Y = 6 - 2X

  • The relationship between Y and X is determined; for each value of X there is one and only one value of Y [function]

  • Substitute a value of X into the equation to determine the value of Y

  • Values of X and Y may be positive or negative, for many uses in economics the values are positive [we use the NE quadrant]

Principles of Microeconomics


Slide7 l.jpg

Y>0

The X axis

[horizontal]

+3

+2

+1

X<0

X > 0

-3

-2

-1

+1

+2

+3

-1

-2

-3

The Y axis [vertical]

Y<0

Equations -- Graphs [Cartesian system]

The North East Quadrant

(NE), where X > 0, Y > 0

{both X and Y are positive

numbers}

(X,Y) where

X<0, Y>0

(X,Y)

where X>0 and Y<0

(X,Y)

where X<0 and Y<0

(Left click mouse to add material)

Principles of Microeconomics


Slide8 l.jpg

6

5

3

2

1

1

2

3

4

5

6

When the values of the independent and dependent variables

are positive, we use the North East quadrant

(Left click mouse to add material)

Go to the right {+3} units and

up {+5} units!

(X, Y)

(3, 5)

Right {+1} one

and up {+6} six

(1,6)

(5, 1)

(2.5, 3.2)

Right 5 and up 1

to the right 2.5 units

and up 3.2 units

Principles of Microeconomics


Slide9 l.jpg

A

6

5

4

3

2

1

B

1

2

3

4

5

6

Given the relationship, Y = 6 - 2X,

(Left click mouse to add

material)

when X = 0 then Y = 6

[this is Y-intercept]

Y

sets of (X, Y)

A line that slopes from

upper left to lower right

represents an inverse or

negative relationship, when the

value of X increases, Y

decreases!

(0, 6)

when X = 1

then Y = 4

(1, 4)

(2, 2)

(3, 0)

When X = 2, then Y = 2

The relationship for all positive

values of X and Y can be illustrated

by the line AB

When X = 3, Y = 0,

[this is X-intercept]

X

Principles of Microeconomics


Slide10 l.jpg

Y

6

5

4

3

2

1

X

1

2

3

4

5

6

(Left click mouse to add material)

Given a relationship, Y = 6 - .5X

(0,6)

(1,5.5)

(2, 5)

(4,4)

(6,3)

For every one unit increase in

the value of X, Y decreases by

one half unit. The slope of this

function is -.5! The Y-intercept is 6.

What is the X-intercept?

Principles of Microeconomics


Slide11 l.jpg

Y

6

5

rise

+2

4

3

1

2

run

+1

slope = +

2

rise

+1

1

run

+2

X

1

2

3

4

5

6

-1

For a relationship, Y = 1 + 2X

When X=0, Y=1 (0,1)

When X = 1, Y = 3

slope = +2

(1,3)

When X = 2, Y = 5

(2,5)

This function illustrates a positive relationship

between X and Y. For every one unit increase

in X, Y increases by 2 !

for a relationship

Y = -1 + .5X

This function shows that for a 1

unit increase in X, Y increases

one half unit

(Left click mouse to add material)

Principles of Microeconomics


Problem l.jpg
Problem

  • Graph the equation: Y = 9 - 3X

    • What is the Y intercept? The slope?

    • What is the X intercept? Is this a positive (direct) relationship or negative (inverse)?

  • Graph the equation Y = -5 + 2X

    • What is the Y intercept? The slope?

    • What is the X intercept? Is this a positive (direct) relationship or negative (inverse)?

Principles of Microeconomics


Equations in economics l.jpg
Equations in Economics

  • The quantity [Q] of a good that a person will buy is determined partly by the price [P] of the good. [Note that there are other factors that determine Q.]

  • Q is a function of P, given a Price the quantity of goods purchased is determined. Q = fp(P)

  • A function is relationship between two sets in which there is one and only one element in the second set determined by each element in the first set.

Principles of Microeconomics


Relationship cont l.jpg
Relationship [cont . . . ]

  • Q = fp(P) {Q is a function of P}

  • Example: Q = 220 - 5P

  • If P = 0, then Q = 220

  • If P = 1, then Q = 215

  • for each one unit increase in the value of P, the value of Q decreases by 5

Principles of Microeconomics


Q 220 5p l.jpg
Q = 220 - 5P

  • This is an inverse or negative relationship

    • as the value of P increases, the value of Q decreases

  • the “Y intercept” is 220, this is the value of Q when; P = 0

  • the “X intercept” is 44, this is the value of P when Q = 0

  • This is a “linear function,” i.e. a straight line

  • The “slope” of the function is -5

    • for every 1 unit change in P, Q changes by 5 in the opposite direction

Principles of Microeconomics


Slide16 l.jpg

The equation provides the information to construct a table.

However, it is not possible to make a table to include every

possible value of P. The table contains “discrete” data and does

not show all possible values!

Principles of Microeconomics


Slide17 l.jpg

55

50

45

40

35

30

At P=$30,

Q = 70

25

20

15

10

At a price of $10, the

the quantity is

$5

Demand

70

170

40

80

120

160

200

240

280

For the relationship, Q = 220 - 5P,

the relationship can be graphed ...

PRICE

When the price is $44, 0 unit will be bought;

at a price of $0, 220 units will be bought.

44

Notice that we have drawn the

graph “backwards,” P{independent}

variable is placed on the Y-axis.

This is done because we eventually

want to put supply on the same

graph and one or the other must be

reversed! Sorry!

QUANTITY

Principles of Microeconomics

(Left click mouse to add material)


Slopes and shifts l.jpg
Slopes and Shifts

  • Economists are interested in how one variable{the independent}“causes” changes in another variable {the dependent}

    • this is measured by the slope of the function

  • Economists are also interested in changes in the relationship between the variables

    • this is measured by “shifts” of the function

Principles of Microeconomics


Slope of a function or line l.jpg
Slope of a function or “line”

  • The slope measures the change in the dependent variable that will be “caused” by a change in the independent variable

  • When, Y = a ± m X; m is the slope

Principles of Microeconomics


Slide20 l.jpg

Y

6

5

4

DY= -1

3

2

1

DX = 2

X

1

2

3

4

5

6

rise

slope is

run

Slope of a Line

Y = 6 -.5X

as the value of X

increases from 2 to 4,

the value of Y

decreases from

5 to 4

DY is the rise [or change in Y caused by DX]{in this case, -1}

so, slope is -1/2 or -.5

DX is the run {+2},

Principles of Microeconomics


Shifts of function l.jpg
Shifts of function

  • When the relationship between two variables changes, the function or line “shifts”

  • This shift is caused by a change in some variable not included in the equation

    • [the equation is a polynomial]

  • A shift of the function will change the intercepts [and in some cases the slope]

Principles of Microeconomics


Slide22 l.jpg

(Left click mouse to add material)

Y

6

Shifts

right

5

4

shifts

left

3

an increase in the

function would

represent an

increase in the

intercept [from 6

to a larger number]

A decrease in the

function would be

Y’ = 4 - .5X

2

1

X

1

2

3

4

5

6

the function shifts and its

slope also changes

Just the slope

changes

{in this case, an increase in the absolute

value of .5 to -1.8}

Y” = 6 - 1.8X [x intercept = 3.3]

Given the function Y = 6 - .5X,

Principles of Microeconomics


Shifts in functions l.jpg
Shifts in functions

  • In Principles of Economics most functions are graphed in 2-dimensions, this means we have 2 variables. [The dependent and independent]

  • Most dependent variables are determined by several or many variables, this requires polynomials to express the relationships

  • a change in one of these variables which is not shown on a 2-D graph causes the function to “shift”

Principles of Microeconomics


Slope and production l.jpg
Slope and Production

  • The output of a good is determined by the amounts of inputs and technology used in production

  • example of a case where land is fixed and fertilizer is added to the production of tomatoes.

  • with no fertilizer some tomatoes, too much fertilizer and it destroys tomatoes

Principles of Microeconomics


Slide25 l.jpg

12

11

10

TPf

9

8

7

6

5

4

3

2

1

1

2

3

4

5

6

7

8

9

The maximum output of T possible with all inputs

and existing technology is 10 units with 6 units of F

tons of

tomatoes

With the 3rd unit of F,

T increases to 9

With 2 units of F, the output of T

increases to 8

With 1 unit of Fertilizer [F], we get

6 tons

The increase in tomatoes [DT] “caused” by DF

is +3, this is the slope

With no fertilizer we get 3 tons of tomatoes

use of more F causes the tomatoes to “burn” and

output declines

(Left click mouse to add material)

FERTILIZER

Principles of Microeconomics


Slope and marginal product l.jpg
Slope and Marginal Product

  • Since the output of tomatoes [T] is a function of Fertilizer [F] , the other inputs and technology we are able to graph the total product of Fertilizer [TPf]

  • From the TPf, we can calculate the marginal product of fertilizer [MPf]

  • MPf is the DTPf “caused” by theDF

Principles of Microeconomics


Slide27 l.jpg

12

11

10

TPf

9

8

7

6

3

5

rise = +3

4

+3

3

2

run=1

1

1

2

3

4

5

6

7

8

9

Given: T = f (F, . . . ), MPf = [DTPf/DF]

DTPf = +1, DF = +3; +1/+3 @ .33

[this is an approximation because DF>1]

DTPf = -1, DF = +2; -1/+2 = -.5

Fertilizer [F] Tomatoes [T]

0 3

MPf [slope]

3

{technically,

this is

between 0

and the first

unit of F}

1 6

2 8

2

3 9

1

6 10

.33

rise/run =+3

8 9

-.5[a negative slope!]

DTPf = +3, DF = +1; +3/+1 = 3 [slope = +3]

DTPf = +2, DF = +1; +2/+1 = 2

DTPf = +1, DF = +1; +1/+1 = 1

Principles of Microeconomics


Slide28 l.jpg

-220

-220

-5

-5

-5

1

44 -

Q = 1P

5

Given a functional relationship such as: Q = 220 - 5P,

we can express the equation for P as a function of Q

Think of an equation as a “balance scale,” what you do to one side

of the equation you must do to the other in order to maintain balance

Q = 220 - 5P

subtract 220 from both sides

-220 + Q = -5P

divide every term in both sidesby -5

or,P = 44 - .2 Q

The equation P = 44 - .2Q

is the same as Q = 220 - .5P

(Left click mouse to add material)

Principles of Microeconomics


How do economists estimate relationships l.jpg
How do economists estimate relationships?

  • Humans behavioral relationships are:

    • modeled on the basis of theories

    • models are verified through empirical observations and statistical methods

  • The relationships are estimates that represent populations {or distributions} not specific individuals or elements

Principles of Microeconomics


An example l.jpg
An Example

  • Hypothesis: the amount of good X [Q] that Susan purchases is determined by the price of the good [Px], Susans’s income [Y], prices of other related goods [Pr] and Susan’s preferences.

  • Q = fi (Px,Y, Pr, preferences, . . .)

    • [. . . indicates there are other variables that are not included in the equation]

Principles of Microeconomics


Model of relationship l.jpg
Model of Relationship

  • Q = fi (Px,Y, Pr, preferences, . . .) acts a a model to represent the relationships of each independent variable to Q [dependent variable]

  • For simplicity, the relationship is described as “linear.” If the relationship were believed not to be linear, with a bit more effort we might construct a “nonlinear model.”

Principles of Microeconomics


Empirical verification l.jpg
Empirical verification

  • To test the model, we would like to observe Susan’s buying pattern.

  • If Px,Y, Pr and preferences were all changing at the same time, we would use a multivariate analysis called “multiple regression.”For simplicity we have been lucky enough to find a period where only Px has changed. Y, Pr and preferences have remained unchanged over the period in which we observe Susan’s purchases

Principles of Microeconomics


Slide33 l.jpg

Price of good X

18

16

14

12

10

8

6

4

2

2

4

6

8

10

12

14

16

18

20

22

Quantity per week

During a 5 week period,

Susan was observed

making the following

purchases

Data from these observations

can be plotted on the graph

Clearly there is a pattern, however it is not a perfect relationship.

Through statistical inference we can estimate some general

characteristics about the relationship

Principles of Microeconomics


Slide34 l.jpg

Given the observed data about Susan’s purchases:

Price of good X

18

16

( Q= 10, P= $15)

14

(15, 11)

12

(20,10)

10

8

(22,7)

6

(22,6)

4

2

2 4 6 8 10 12 14 16 18 20 22 24 26

Quantity per week

We can estimate a line that minimizes the square of the

difference that each point [that represents two variables]

lies off the estimated line.

P = 23 - .75Q may be written

Q = 30.667- 1.333P

No single point may lie

the line, but the line is an

estimate of the relationship

P = 23 - .75Q is our estimate

of the relationship between the

price and the quantity that Susan

purchases each week, ceteris paribus or

all other things equal

Principles of Microeconomics


Slide35 l.jpg

Given the observed data about Susan’s purchases:

Price of good X

18

16

14

12

10

P = 10

8

P = 6

6

4

Q = 22.67

2

Q = 17.37

2 4 6 8 10 12 14 16 18 20 22 24 26

Quantity per week

and our estimated function: P = 23 - .75Q or Q = 30.67 - 1.33P,

we would predict that at a price of $10 Susan would

purchase about 17.37 units, [Q = 30.67 - 1 .33 P,

P = 10 so Q =17.37]

We observed that Susan bought 20 units

when the price was $10 so estimate is off

by a small amount [-2.63 units]

At a price of $6 our equation predicts

that 22.67 units will be purchased

Since we observed that she

purchased 22, we are off

by .67 units

our estimates are not perfect, but they

give an approximation of the relationship

Principles of Microeconomics


Statistical estimates l.jpg
Statistical Estimates

  • The estimates are not “perfect” but they provide reasonable estimates

  • There are many statistical tools that measure the confidence that we have in out predictions

    • these include such things as correlation, coefficient of determination, standard errors, t-scores and F-ratios

Principles of Microeconomics


Slope calculus l.jpg
Slope & Calculus

  • In economics we are interested in how a change in one variable changes another

    • How a change in price changes sales. How a change in an input changes output. How a change in output changes cost. etc.

  • The rate of change is measured by the slope of the functional relationship

    • by subtraction the slope was calculated as rise over run where rise = DY = Y1 - Y2 and run = DX = X1 - X2,

Principles of Microeconomics


Derivative there are still more slides on this topic l.jpg
DerivativeThere are still more slides on this topic

  • When we have a nonlinear function, a simple derivative can be used to calculate the slope of the tangent to the function at any value of the independent variable

  • The notation for a derivative is written:

Principles of Microeconomics


Summary l.jpg

dY

dX

dY

dX

Summary

  • a derivative is the slope of a tangent at a point on a function

  • is the rate of change, it measures the change in Y caused by a change in X as the change in X approaches 0

  • in economics jargon, [the slope or rate of change] is the “marginal”

Principles of Microeconomics


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