huw owens - university of manchester : february 061 tx-1037 mathematical techniques for managers dr...

Huw Owens - University of Manchester : February 06 1

TX-1037 Mathematical Techniques for TX-1037 Mathematical Techniques for ManagersManagers

Dr Huw OwensRoom B44 Sackville Street Building

Telephone Number 65891


IntroductionIntroduction

• In Lecture 1 we looked at,• Coordinates and Graphs• Fractions• Variables and Functions• Linear Functions• Power Functions• Sketching Functions• Algebra

• Factors and multiplying out brackets• Accuracy• Powers and Indices


• Functions of more than One Variable• Economic Variables and Functions• Total and Average Revenue• Total and Average Cost• Profit• Production Functions, Isoquants and the average

Product of Labour


Equations in EconomicsEquations in Economics

• Lecture objectives• Understand how equations are used in economics• Rewrite and solve equations• Substitute expressions• Solve simple linear demand and supply equations to

find market equilibrium• Carry out cost-volume-Profit analysis• Identify the slope and intercept of a line• Plot the budget constraint to obtain the budget line


Variables and functionsVariables and functions

• Variable: a quantity represented by a symbol that can take different possible values (variable names x and y are often used).

• Constant: a quantity whose value is fixed, even if we do not know its numerical amount (letters commonly used to represent constants are: a,b,c,k).

• Function: a systematic relationship between pairs of values of the variables, written y=f(x).

• If one variable, y, changes in a systematic way as another variable, x, changes we say y is a function of x. The mathematical notation for this is

• y=f(x), where the letter f is used to denote a function.• If there is more than one functional relationship we can

indicate they are different by using different letters, such as g or h.

• For example, y=g(x), which is read as “y is a function of x”


Substitution of x-valuesSubstitution of x-values

• A function gives a general rule for obtaining values of y from values of x. An example is

• y=4x+5, where 4x means 4*x (by convention we omit the multiplication sign).

• To evaluate the function for a particular value of x, say x= 6

• y=4*6+5, y=29• Substituting different values into the function gives us

different points on a graph.• As the function tells us how to obtain y values from any x

values, y is said to be dependent on x, and x is known as the independent variable.

• The independent variable is plotted on the horizontal axis and the independent variable is plotted on the vertical axis.


Linear functionsLinear functions

• If the relationship between x and y takes the form y=6x

x 0 5 10

y 0 30 60

y=6x

0

30

60

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12

x

y y=6x



• Proportional relationship: each y value is the same amount times the corresponding x value, so all points lie on a straight line through the origin.

• Linear function: a relationship in which all the pairs of values form points on a straight line.

• In general, a function of the form y=bx represents a straight line passing through the origin.

• Shift: a vertical movement upwards or downwards of a line or curve.

• Adding a constant to a function shifts the function vertically upwards by the amount of the constant.

• For example, y=6x+20 has y values that are 20 more than those of the previous function of every value of x.



• Intercept: the value at which a function cuts the y-axis.• Remember – a function with a term just in x and perhaps a

constant is a linear function. It has the general form • y=ax+b

y=6x+20

0

30

60

20

50

80

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8 10 12

x

y

y=6x

y=6x+20


Power functionsPower functions

• Power: an index indicating the number of times the item to which is applied is multiplied by itself.

• For example, y = x2 or z = 7x2

• If we evaluate these functions and substitute x=5 we obtain• y=x2, y = x*x, y = 5*5, y = 25• y = 7x2, y = 7*x*x, y = 7*5*5, y = 175• Functions can have more than one term and one may be a

constant.• For example, y = 140+7x2-2x3 or y=25x2+74• Quadratic function: a function in which the highest power of

x is two. There may also be a term in x and a constant but no other terms.

• Cubic function: a function in which the highest power of x is 3. There may also be terms in x2, x and a constant, but no other terms.


Power Functions – An examplePower Functions – An example

• Sketch and briefly describe the following functions for positive values of x

• y=2x3-50 and y = 14y=6x+20

0-48 -84 -96 -72

0

132

336

624

1008

1500

14 14 14 14 14 14 14 14 14 14 14

-200

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12

x

y

y=2x̂ 3-50x

y=14


Some questionsSome questions

• Sketch the graphs of the functions for values of x between 0 and 10

• y=0.5x• y=0.5x + 6• y=x2

• y=3x2

• Which of them is linear? Which is a proportional relationship? What is the effect of adding a constant term?


Some questionsSome questions

• y=0.5x – Linear, proportional relationship• y=0.5x+6, Linear, non-proportional relationship, shifted up

by constant• y = x2. Quadratic function.• y=3x2. Quadratic function. Some questions

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 56 6.5 7 7.5 8 8.5 9 9.5 10 10.5 110 1 4 9 16 25

3649

6481

100

0 312

2748

75

108

147

192

243

300

0

50

100

150

200

250

300

350

0 2 4 6 8 10 12

x

y

y=0.5x

y=0.5x+6

y=x̂ 2

y=3x̂ 2


Factors and multiplying out bracketsFactors and multiplying out brackets

Factorising: writing an expression as a product that when multiplied out gives the original expression.

E.g. y=6x-3x2, this is a shorthand way of writing y=(6*x)-(3*x*x)

If we divide terms on the right-hand side by3*x we obtain (6*x)/3*x = 2 and (–3*x*x)/(3*x) = -x.

The amount we divide by we call a common factor.So factorising the expression y=6x-3x2 we obtainy=3x(2-x)When two brackets are multiplied together, to remove

them we multiply each term in the second bracket by each term in the first bracket. It is then usual to simplify the result by collecting terms where possible.


Multiplying bracketsMultiplying brackets

• E.g. (a-b)(-c+d) = -ac+ad- -bc-+bd = -ac+ad+bc-bd• E.g. (x-7)(4-3x) = 4x-3x2-28+21x =-3x2+25x-28 • Factorisation is the reverse process to multiplying out

brackets.• It might not be obvious and could require some

intelligent guesswork.• The following standard results of multiplying out

brackets are helpful.• (a+b)2 = a2+2ab+b2

• (a-b)2=a2-2ab+b2

• (a+b)(a-b)=a2-b2

• NOTE: Not every quadratic expression factorises to an expression that contains integer values.


Powers and indicesPowers and indices

• Index or power: a superscript showing the number of times the value to which it is applied is to be multiplied by itself.

• E.g. x3 = x*x*x, x1 = x• When we multiply together two expressions

comprising the same value raised to a power we ADD the indices and raise to that new power.

• E.g. x3*x5 = (x*x*x)*(x*x*x*x*x)=x8

• Using our rule, x3*x5 = x3+5 = x8

• When we divide together two expressions comprising the same value raised to a power we SUBTRACT the indices and raise to that new power.

2353

5

1

*

**

****xx

xx

xxx

xxxxx

x

x


What is an equation?What is an equation?

• Equation: two expressions separated by an equals sign such that what is on the left of the equals sign has the same value as what is on the right.

• Solving equations lets us discover where lines or curves intersect.

• These points are interesting because they often indicate information about equilibrium situations.

• A graphical solution can be obtained by sketching the curves and reading off the x and y values at the point where they cross BUT results only have limited accuracy.


The elimination methodThe elimination method

• Why use elimination?• The graphical method has several drawbacks

• How do you decide suitable axes?• Accuracy of the graphical solution?• Complex problems with > three equations and >

three unknowns?


ExampleExample

• 4x+3y = 11 (1)• 2x+y = 5 (2)• The coefficient of x in equation 1 is 4 and the

coefficient of x in equation 2 is 2• By multiplying equation 2 by 2 we get• 4x+2y = 10 (3)• Subtract equation 3 from equation 1 to get

4x + 3y = 11

minus 4x + 2y = 10

y = 1


ExampleExample

• If we substitute y=1 back into one of the original equations we can deduce the value of x.

• If we substitute into equation 1 then• 4x+3(1)=11• 4x=11-3• 4x=8• x=2• To check this put substitute these values (2,1) back

into one of the original equations• 2*2+1 = 5


Summary of the method of eliminationSummary of the method of elimination

• Step 1 – Add/subtract a multiple of one equation to/from a multiple of the other to eliminate x.

• Step 2- Solve the resulting equation for y.• Substitute the value of y into one of the original

equations to deduce x.• Step 4 – Check that no mistakes have been made by

substituting both x and y into the other original equation.


Example involving fractionsExample involving fractions

• Solve the system of equations• 3x+2y =1 (1)• -2x + y = 2 (2)

• Solution• Step 1 - Set the x coefficients of the two equations to the same

value. We can do this by multiplying the first equation by 2 and the second by 3 to give

• 6x+4y = 2 (3)• -6x+3y = 6 (4)• Add equations 3 and 4 together to cancel the x coefficients• 7y = 8• y=8/7• Step three substitute y = 8/7 into one of the original equations• 3x+2*8/7=1


Example Example

• 3x=1-16/7• 3x=-9/7• x = -9/7*1/3• x= -3/7• The solution is therefore x= -3/7, y= 8/7• Step 4 check using equation 2• -2*(-3/7)+8/7 = 2• 6/7+8/7 = 2• 14/7 = 2• 2=2


ProblemsProblems

• 1) Solve the following using the elimination method• 3x-2y = 4• x-2y =2• 2) Solve the following using the elimination method• 3x+5y = 19• -5x+2y = -11


Special CasesSpecial Cases

• Solve the system of equations • x-2y = 1• 2x-4y=-3• The original system of equations does not have a

solution. Why?• Solve the system of equations• 2x-4y = 1• 5x-10y = 5/2• This original system of equations does not have a

unique solution



• There can be a unique solution, no solution or infinitely many solutions. We can detect this in Step 2.

• If the equation resulting from elimination of x looks like the following then the equations have a unique solution

• If the elimination of x looks like the following then the equations have no solutions

Any non-zero number

Anynumbery =*

zero Any non-zeronumbery =*



• If the elimination of x looks like the following then the equations have infinitely many solutions

zero zeroy =*


Elimination Strategy for three equations with three Elimination Strategy for three equations with three unknownsunknowns

• Step 1 – Add/Subtract multiples of the first equation to/from multiples of the second and third equations to eliminate x. This produces a new system of the form

• ?x + ?y + ?z = ?• ?y+?z = ?• ?y+?z =?• Step 2 – Add/subtract a multiple of the second

equation to/from a multiple of the third to eliminate y. This produces a new system of the form

• ?x + ?y + ?z = ?• ?y+?z = ?• ?z = ?


• Step 3 – Solve the last equation for z. Substitute the value of z into the second equation to deduce y. Finally, substitute the values of both y and z into the first equation to deduce x.

• Step 4 – Check that no mistakes have been made by substituting the values of x,y and z into the original equations.

• Example – Solve the equations• 4x+y+3z = 8 (1)• -2x+5y+z = 4 (2)• 3x+2y+4z = 9 (3)• Step 1 – To eliminate x from the second equation

multiply it by 2 and then add to equation 1


• To eliminate x from the third equation we multiply equation 1 by 3, multiply equation 3 by 4 and subtract

• Step 2 – To eliminate y from the new third equation (5) we multiply equation 4 by 5, multiply equation 5 by 11 and add

• This gives us z = 1. Substitute back into equation 4. This gives us y = 1.

• Finally substituting y=1 and z=1 into equation 1 gives the solution x=1, y=1, z=1

• Step 4 Check the original equations give• 4(1)+1+3(1) = 8• -2(1)+5(1)+1=4• 3(1)+2(1)+4(1)=9• respectively


Practice ProblemsPractice Problems

• Sketch the following lines on the same diagram• 2x-3y=6• 4x-6y=18• x-3/2y=3• Hence comment on the nature of the solutions of the

following system of equations• A)

• 2x-3y = 6• x-3/2y=3

• B) • 4x-6y=18• x-3/2y=3


Supply and Demand AnalysisSupply and Demand Analysis

• At the end of this lecture you should be able to • Use the function notation, y=f(x)• Identify the endogenous and exogenous variables

in the economic model.• Identify and sketch a linear demand function.• Identify and sketch a linear supply function.• Determine the equilibrium price and quantity for a

single-commodity market both graphically and algebraically.

• Determine the equilibrium price and quantity for a multi-commodity market by solving simultaneous linear equations


MicroeconomicsMicroeconomics

• Microeconomics is concerned with the analysis of the economic theory and policy of individual firms and markets.

• This section focuses on one particular aspect known as market equilibrium in which supply and demand balance.

• What is a function?• A function f, is a rule which assigns to each incoming

number, x, a uniquely defined out-going number, y.• A function may be thought of as a “black-box” which

performs a dedicated arithmetic calculation.• An example of this may be the rule “double and add

3”.


• For example, a second function might be• g(x) = -3x+10• We can subsequently identify the respective functions

by f and g


• We can write this rule as –• y=2x+3• Or f(x)=2x+3

Double and Add 3

5 13

Double and Add 3

-17 -31

• If in a piece of economic theory, there are two or more functions we can use different labels to refer to each one.

f(5)=13

f(-17)


Independent and dependent variablesIndependent and dependent variables

• The incoming and outgoing variables are referred to as the independent and dependent variables respectively. The value of y depends on the actual value of x that is fed into the function.

• For example, in microeconomics the quantity demanded, Q, of a good depends on the market price, P. This may be expressed as Q = f(P).

• This type of function is known as a demand function.• For any given formula for f(P) it is a simple matter to

produce a picture of the corresponding demand curve on paper.

• Economists plot P on the vertical axis and Q on the horizontal axis.


But first a ProblemBut first a Problem

• Evaluate• f(25)• f(1)• f(17)• g(0)• g(48)• g(16)• For the functions• f(x) = -2x +50• g(x) = -1/2x+25• Do you notice any connection between f and g?


• P=g(Q)• Thus the two functions f and g are said to be inverse

functions.• The above form P=g(Q), the demand function, tells us

that P is a function of Q but does not give us any precise details.

• If we hypothesize that the function is linear – • P = aQ+b (for some appropriate constants called

parameters a and b)• The process of identifying real world features and

making appropriate simplifications and assumptions is known as modelling.

• Models are based on economic laws and help to explain the behaviour of real, world situations.


• A graph of a typical linear demand function may be seen below.

• Demand usually falls as the price of the good rises and so the slope of the line is negative.

• In mathematical terms P is said to be a decreasing function of Q.

• So a<0 “a is less than zero” and b>0 “b is greater than zero”

b

P

Q


ExampleExample

• Sketch the graph of the demand function P=-2Q+50• Hence or otherwise, determine the value of • (a) P when Q=9• (b) Q when P=10• Solution• (a) P = –2*9+50, P=32• (b) 10 = -2Q+50, -40 = -2Q, 20 = Q• Sketch a graph of the demand function P = -3Q+75• Hence, or otherwise, determine the value of • (a) P when Q=23• (b) Q when P=18• Solution• (a) P = -69+75, P = 6• (b) 18 = -3Q+75, -57 = -3Q, 19 = Q


• We’ve so far looked at a crude model of consumer demand assuming that the quantity sold is based only on the price.

• In practice other factors are required such as the incomes of the consumers Y, the price of substitute goods PS, the price of complementary goods PC, advertising expenditure A, and consumer tastes T.

• A substitute good is one which could be consumed instead of the good under consideration. (e.g. buses and taxis)

• A complementary good is one which is used in conjunction with other goods (e.g. DVDs and DVD players).

• Mathematically, we say that Q is a function of P, Y, PS,PC, A and T.


Endogenous and exogenous variablesEndogenous and exogenous variables

• This is written as Q=f(P,Y,PS,PC,A,T)• In terms of our “black box” diagram

• Any variables which are allowed to vary and are determined within the model are known as endogenous variables (Q and P).

• The remaining variables are called exogenous since they are constant and are determined outside the model.

f Q

P

YPS

PT

AT


Inferior and superior goodsInferior and superior goods

• An inferior good is one whose demand falls as income rises (e.g. coal vs central heating)

• A superior good is one whose demand rises as income rises (e.g. cars and electrical goods).

• Problem• Describe the effect on the demand curve due to an

increase in• (a) the price of substitutable goods, Ps• (b) the price of complementary goods, Pc• (c) advertising expenditure


The supply functionThe supply function

• The supply function is the relation between the quantity, Q, of a good that producers plan to bring to the market and the price, P, of the good.

• A typical linear supply curve is indicated in the diagram below.

• Economic theory indicates that as the price rises so does the supply. (Mathematically P is an increasing function of Q)

b

P

Q

Supply curve

Demand curve

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