Linear Models And Systems Of Linear Equations

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Chapter 1Linear Models andSystems of LinearEquations

Contents1 Linear Models and Systems of Linear Equations1.1 Mathematical Models . . . . . . . . . . . . . . . . .1.1.1 Functions . . . . . . . . . . . . . . . . . . . .1.1.2 Mathematical Modeling . . . . . . . . . . . .1.1.3 Cost, Revenue, and Profits . . . . . . . . . .1.1.4 Supply and Demand . . . . . . . . . . . . . .1.1.5 Straight-Line Depreciation. . . . . . . . . . .1.2 Systems of Linear Equations . . . . . . . . . . . . . .1.2.1 Two Linear Equations in Two Unknowns . .1.2.2 Decision Analysis . . . . . . . . . . . . . . . .1.2.3 Supply and Demand Equilibrium . . . . . . .1.2.4 Enrichment: Decision Analysis Complications1.022569112122232426

1.1Mathematical ModelsAugustin Cournot, 1801-1877The first significant work dealing with the application ofmathematics to economics was Cournot’s Researches intothe Mathematical Principles of the Theory of Wealth, published in 1836. It was Cournot who originated the supplyand demand curves that are discussed in this section. IrvingFisher, a prominent economics professor at Yale Universityand one of the first exponents of mathematical economics inthe United States, wrote that Cournot’s book “seemed a failure when first published. It was far in advance of the times.Its methods were too strange, its reasoning too intricate forthe crude and confident notions of political economy thencurrent.”Application: Cost, Revenue, and Profit ModelsA firm has weekly fixed costs of 80,000 associated with themanufacture of dresses that cost 25 per dress to produce.The firm sells all the dresses it produces at 75 per dress.Find the cost, revenue, and profit equations if x is the numberof dresses produced per week. See Example 3 for the answer.We will first review some basic material on functions. An introduction to the mathematical theory of the business firm with somenecessary economics background is provided. We study mathematical business models of cost, revenue, profit, and depreciation, andmathematical economic models of demand and supply. We will onlyconsider linear relationships, so you may wish to review materiallocated in the Algebra Review chapter on straight lines.1.1.1FunctionsMathematical modeling is an attempt to describe some part of thereal world in mathematical terms. Our models will be functionsthat show the relationship between two or more variables. Thesevariables will represent quantities that we wish to understand or describe. Examples include the price of gasoline, the cost of producingcereal or the number of video games sold. The idea of representingthese quantities as variables in a function is central to our goal ofcreating models to describe their behavior. We will begin by reviewing the concept of functions. In short, we call any rule that assignsor corresponds to each element in one set precisely one element inanother set a function.

1.1 Mathematical Modeling1-3For example, suppose you are going a steady speed of 40 miles perhour in a car. In one hour you will travel 40 miles; in two hours youwill travel 80 miles; and so on. The distance you travel depends on(corresponds to) the time. Indeed, the equation relating the variablesdistance (d), velocity (v), and time (t), is d v · t. In our example,we have a constant velocity of v 40, so d 40 · t. We can viewthis as a correspondence or rule: Given the time t in hours, the rulegives a distance d in miles according to d 40 · t. Thus, given t 3,d 40·3 120. Notice carefully how this rule is unambiguous. Thatis, given any time t, the rule specifies one and only one distance d.This rule is therefore a function; the correspondence is between timeand distance.Often the letter f is used to denote a function. Thus, using theprevious example, we can write d f (t) 40 · t. The symbol f (t)is read “f of t.” One can think of the variable t as the “input” andthe value of the variable d f (t) as the “output.” For example, aninput of t 4 results in an output of d f (4) 40 · 4 160 miles.The following gives a general definition of a function.Definition of a FunctionA function f from D to R is a rule that assigns to eachelement x in D one and only one element y f (x) in R. SeeFigure 1.1.1.Figure 1.1.1The caption is here, if neededFigure 1.1.2The set D in the definition is called the domain of f . We mightthink of the domain as the set of inputs. We then can think of thevalues f (x) as outputs. The set of outputs, R is called the range off.Another helpful way to think of a function is shown in Figure 1.1.2. Here the function f accepts the input x from the conveyorbelt, operates on x, and outputs (assigns) the new value f (x).The letter representing elements in the domain is called the independent variable, and the letter representing the elements inthe range is called the dependent variable. Thus, if y f (x), xis the independent variable, and y is the dependent variable, sincethe value of y depends on x. In the equation d 40t, we can writed f (t) 40t with t as the independent variable. The dependentvariable is d, since the distance depends on the spent time t traveling.We are free to set the independent variable t equal to any numberof values in the domain. The domain for this function is t 0 sinceonly nonnegative time is allowed.Note that the domain in an application problem will always bethose values that are allowed for the independent variable in theparticular application. This often means that we are restricted tonon-negative values or perhaps we will be limited to the case of wholenumbers only, as in the next example:

1.1 Mathematical Modeling1-4Example 1Steak Specials A restaurant serves a steak special for 12. Writea function that models the amount of revenue made from selling thesespecials. How much revenue will 10 steak specials earn?Solution:We first need to decide if the independent variable is the price ofthe steak specials, the number of specials sold, or the amount ofrevenue earned. Since the price is fixed at 12 per special and revenuedepends on the number of specials sold, we choose the independentvariable, x, to be the number of specials sold and the dependentvariable, R f (x) to be the amount of revenue. Our rule will beR f (x) 12x where x is the number of steak specials sold and Ris the revenue from selling these specials in dollars. Note that x mustbe a whole number, so the domain is x 0, 1, 2, 3, . . . To determinethe revenue made on selling 10 steak specials, plug x 10 into themodel:R f (10) 12(10) 120 So the revenue is 120.TTechnology Option. You may wish to see Technology Note 1 forthe solution to the question using the graphing calculator.Recall (see Appendix A) that lines satisfy the equation y mx b. Actually, we can view this as a function. We can set y f (x) mx b. Given any number x, f (x) is obtained by multiplying x bym and adding b. More specifically, we call the function y f (x) mx b a linear function.Definition of Linear FunctionA linear function f is any function of the formy f (x) mx bwhere m and b are constants.Example 2a.b.c.d.Linear Functions Which of the following functions are linear?y 0.5x 125y 2x 10y 1/x 2y x2Solution: a. This is a linear function. The slope is m 0.5 and the y-interceptis b 12.b. Rewrite this function first as,5y 2x 105y 2x 10y (2/5)x 2Now we see it is a linear function with m 2/5 and b 2.

1.1 Mathematical Modeling1-5c. This is not a linear function. Rewrite 1/x as x 1 and this showsthat we do not have a term mx and so this is not a linear function.d. Here x is raised to the second power and so this is not a linear function.1.1.2Mathematical ModelingWhen we use mathematical modeling we are attempting to describesome part of the real world in mathematical terms, just as we havedone for the distance traveled and the revenue from selling meals.There are three steps in mathematical modeling: formulation, mathematical manipulation, and evaluation.FormulationFirst, on the basis of observations, we must state a question or formulate a hypothesis. If the question or hypothesis is too vague,we need to make it precise. If it is too ambitious, we need to restrict it or subdivide it into manageable parts. Second, we need toidentify important factors. We must decide which quantities andrelationships are important to answer the question and which canbe ignored. We then need to formulate a mathematical description.For example, each important quantity should be represented by avariable. Each relationship should be represented by an equation,inequality, or other mathematical construct. If we obtain a function,say, y f (x), we must carefully identify the input variable x andthe output variable y and the units for each. We should also indicatethe interval of values of the input variable for which the model isjustified.Mathematical ManipulationAfter the mathematical formulation, we then need to do some mathematical manipulation to obtain the answer to our original question.We might need to do a calculation, solve an equation, or prove atheorem. Sometimes the mathematical formulation gives us a mathematical problem that is impossible to solve. In such a case, we willneed to reformulate the question in a less ambitious manner.EvaluationNaturally, we need to check the answers given by the model withreal data. We normally expect the mathematical model to describeonly a very limited aspect of the world and to give only approximateanswers. If the answers are wrong or not accurate enough for our purposes, then we will need to identify the sources of the model’s shortcomings. Perhaps we need to change the model entirely, or perhapswe need to just make some refinements. In any case, this requiresa new mathematical manipulation and evaluation. Thus, modelingoften involves repeating the three steps of formulation, mathematicalmanipulation, and evaluation.

1.1 Mathematical Modeling1-6We will next create linear mathematical models by find equationsthat relate cost, revenue, and profits of a manufacturing firm to thenumber of units produced and sold.1.1.3Cost, Revenue, and ProfitsAny manufacturing firm has two types of costs: fixed and variable.Fixed costs are those that do not depend on the amount of production. These costs include real estate taxes, interest on loans, somemanagement salaries, certain minimal maintenance, and protectionof plant and equipment. Variable costs depend on the amount ofproduction. They include the cost of material and labor. Total cost,or simply cost, is the sum of fixed and variable costs:cost (variable cost) (fixed cost).Let x denote the number of units of a given product or commodity produced by a firm. (Notice that we must have x 0.) The unitscould be bales of cotton, tons of fertilizer, or number of automobiles.In the linear cost model we assume that the cost m of manufacturing one unit is the same no matter how many units are produced.Thus, the variable cost is the number of units produced times thecost of each unit:variable cost (cost per unit) (number of units produced) mxIf b is the fixed cost and C(x) is the cost, then we have thefollowing:C(x) cost (variable cost) (fixed cost) mx bNotice that we must have C(x) 0. In the graph shown in Figure 1.1.3, we see that the y-intercept is the fixed cost and the slopeis the cost per item.Figure 1.1.3CONNECTION*What Are Costs? Isn’t it obvious what the costs to a firmare? Apparently not. On July 15, 2002, Coca-Cola Companyannounced that it would begin treating stock-option compensation as a cost, thereby lowering earnings. If all companiesin the Standard and Poors 500 stock index were to do thesame, the earnings for this index would drop by 23%.* The Wall Street Journal, July 16, 2002.In the linear revenue model we assume that the price p of aunit sold by a firm is the same no matter how many units are sold.(This is a reasonable assumption if the number of units sold by the

1.1 Mathematical Modeling1-7firm is small in comparison to the total number sold by the entireindustry.) Revenue is always the price per unit times the number ofunits sold. Let x be the number of units sold. (For convenience, wealways assume that the number of units sold equals the number ofunits produced.) Then, if we denote the revenue by R(x),R(x) revenue (price per unit) (number sold) pxSince p 0, we must have R(x) 0. Notice in Figure 1.1.4. thatthe straight line goes through (0, 0) because nothing sold results inno revenue. The slope is the price per unit.Figure 1.1.4Connection: What Are Revenues?The accounting practices of many telecommunications companies such as Cisco and Lucent, have been criticized for whatthe companies consider revenues. In particular, these companies have loaned money to other companies, which then usethe proceeds of the loan to buy telecommunications equipment from Cisco and Lucent. Cisco and Lucent then bookthese sales as “revenue.” But is this revenue?Regardless of whether our models of cost and revenue are linearor not, profit P is always revenue less cost. ThusP profit (revenue) (cost) R CRecall that both cost C(x) and revenue R(x) must be nonnegativefunctions. However, the profit P (x) can be positive or negative.Negative profits are called losses.Let’s now determine the cost, revenue, and profit equations for adress-manufacturing firm.Example 3Cost, Revenue, and Profit Equations A firm has weekly fixedcosts of 80,000 associated with the manufacture of dresses that cost 25 per dress to produce. The firm sells all the dresses it producesat 75 per dress.a. Find the cost, revenue, and profit equations if x is the numberof dresses produced per week.b. Make a table of values for cost, revenue, and profit for productionlevels of 1000, 1500 and 2000 dresses and discuss what is the tableof numbers telling you.Solution:a. The fixed cost is 80,000 and the variable cost is 25x. So

1.1 Mathematical Modeling1-8C (variable cost) (fixed cost) mx b 25x 80, 000See Figure 1.1.5a. Notice that x 0 and C(x) 0.The revenue is just the price 75 that each dress is sold multipliedby the number x of dresses sold. SoR (price per dress) (number sold) px 75xSee Figure 1.1.5b. Notice that x 0 and R(x) 0. Also notice thatif there are no sales, then there is no revenue, that is, R(0) 0.Profit is always revenue less cost. SoP (revenue) (cost) R C (75x) (25x 80, 000) 50x 80, 000See Figure 1.1.5c. Notice in Figure 1.1.5c that profits can be negative.10 410 410 420201018188161661414412122Profit: P 50x 80000 100 100Cost: C 25x 800008Revenue: R 75x8664422500 21,0001,5002,0002,5003,000Number of dresses 4 0 6 8 10005001,0001,5002,000Number of dressesFigure r of dressesFigure 1.1.5bFigure 1.1.5cb. To be specific, suppose 1000 dresses are produced and sold. Thenx 1000 andC(1000) 25(1000) 80, 000 130, 000R(1000) 75(1000) 75, 000P (1000) 75, 000 130, 000 55, 000Thus, if 1000 dresses are produced and sold, the cost is 130,000, therevenue is 75,000, and there is a negative profit or loss of 55,000.Doing the same for 1500 and 200 dresses, we have the resultsshown in Table 1.1.

1.1 Mathematical Modeling1-9Number of Dresses Made and Sold1000Cost in dollars130,000Revenue in dollars75,000Profit (or loss) in dollars-55,000Table 0We can see in Figure 1.1.5c or in Table 1.1, that for smaller valuesof x, P (x) is negative; that is, the firm has losses as their costs aregreater than their revenue. For larger values of x, P (x) turns positive and the firm has (positive) profits.1.1.4Supply and DemandIn the previous discussion we assumed that the number of units produced and sold by the given firm was small in comparison to thenumber sold by the industry. Under this assumption it was reasonable to conclude that the price, p, was constant and did not varywith the number x sold. But if the number of units sold by thefirm represented a large percentage of the number sold by the entireindustry, then trying to sell significantly more units could only beaccomplished by lowering the price of each unit. Since we just statedthat the price effects the number sold, you would expect the price tobe the independent variable and thus graphed on the horizontal axis.However, by custom, the price is graphed on the vertical axis andthe quantity x on the horizontal axis. This convention was startedby English economist Alfred Marshall (1842 - 1924) in his importantbook, Principles of Economics. We will abide by this custom in thistext.For most items the relationship between quantity and price is adecreasing function (there are some exceptions to this rule, such ascertain luxury good, medical care and higher eduction, to name afew). That is, for the number of items to be sold to increase, theprice must decrease. We assume now for mathematical conveniencethat this relationship is linear. Then the graph of this equation is astraight line that slopes downward as shown in Figure 1.1.6.We assume that x is the number of units produced and sold bythe entire industry during a given time period and that p D(x) cx d, c 0, is the price of one unit if x units are sold; that is,p cx d is the price of the xth unit sold. We call p D(x) thedemand equation and the graph the demand curve.Figure 1.1.6Estimating the demand equation is a fundamental problem forthe management of any company or business. In the next examplewe consider the situation when just two data points are available andthe demand equation is assumed to be linear.Example 5Finding the Demand Equation Timmins estimated the municipal water demand in Delano, California. He estimated the demandx, measured in acre-feet (the volume of water needed to cover oneacre of ground at a depth of one foot), with price p per acre-foot.

1.1 Mathematical Modeling1-10He indicated two points on the demand curve, (x, p) (1500, 230)and (x, p) (5100, 50). Use this data to estimate the demand curveusing a linear model. Estimate the price when the demand is 100,50)50Figure 1.1.7 shows the two points (x, p) (1500, 230) and (x, p) (5100, 50) that lie on the demand curve. We are assuming that thedemand curve is a straight line. The slope of the line is001,0002,0003,0004,0005,0006,000m xFigure 1.1.750 230 0.055100 1500Now using the point-slope equation for a line with (1500, 230) as thepoint on the line, we havep 230 m(x 1500) 0.05(x 1500)p 0.05x 75 230 0.05x 305When demand is 3000 acre-feet, then x 3000, andp 0.05(3000) 305 155or 155 per acre-foot. Thus, according to this model, if 3000 acre-feet is demanded, the price of each acre-foot will be 155.CONNECTIONDemand for Apartments The figure below shows that during the minor recession of 2001, vacancy rates for apartmentsrose, that is, the demand for apartments decreased. Also notice from the figure that as demand for apartments decreased,rents also decreased. For example, in South Francisco’s SouthBeach area, a two-bedroom apartment that had rented for 3000 a month two years before saw the rent drop to 2100a month.Source: Wall Street Journal, 4-11-02CONNECTION*Demand for Television Sets As sleek flat-panel and highdefinition television sets became more affordable, sales soaredduring the holidays. Sales of ultra-thin, wall-mountable LCDTVs rose over 100% in 2005 to about 20 million sets whileplasma-TV sales rose at a similar pace, to about 5 millionsets. Normally set makers and retailers lower their pricesafter the holidays, but since there was strong demand andproduction shortages for these sets, prices were kept high.* http://biz.yahoo.com/weekend/tvbargain 1.html 1-21-2006

1.1 Mathematical Modeling1-11The supply equation p S(x) gives the price p necessary forsuppliers to make available x units to the market. The graph of thisequation is called the supply curve. A reasonable supply curverises, moving from left to right, because the suppliers of any productnaturally want to sell more if the price is higher. (See Shea6 wholooked at a large number of industries and determined that the supplycurve does indeed slope upward.) If the supply curve is linear, thenas shown in Figure 1.1.8, the graph is a line sloping upward. Notethe positive y-intercept. The y-intercept represents the choke pointor lowest price a supplier is willing to accept.Figure 1.1.8 Example 6Finding the Supply Equation Antle and Capalbo estimated aspring wheat supply curve. Use a mathematical model to determinea linear curve using their estimates that the supply of spring wheatof 50 million bushels at a price of 2.90 per bushel and 100 millionbushels at a price of 4.00 per bushel. Estimate the price when 80million bushels is supplied.Solution: Let x be in millions of bushels of wheat. We are then given two pointson the linear supply curve, (x, p) (50, 2.9) and (x, p) (100, 4).The slope is4 2.9m 0.022100 50The equation is then given by54(100,4)3(50,2.9) /bu2p 0.022x 1.8p 2.9 0.022(x 50)100255075100millions of bushels125150or p 0.022x 1.8. See Figure 1.1.9 and note that the line rises.When supply is 80 million bushels, x 80, and we haveFigure 1.1.9p 0.022(80) 1.8 3.56This gives a price of 3.56 per bushel. CONNECTION*Supply of Cotton On May 2, 2002, the U.S. House of Representatives passed a farm bill that promises billions of dollarsin subsidies to cotton farmers. With the prospect of a greatersupply of cotton, cotton prices dropped 1.36 cents to 33.76cents per pound.* The Wall Street Journal, May 3, 2002.1.1.5Straight-Line Depreciation.Many assets, such as machines or buildings, have a finite useful lifeand furthermore depreciate in value from year to year. For purposesof determining profits and taxes, various methods of depreciation canbe used. In straight-line depreciation we assume that the value6John Shea. 1993. Do supply curves slope up? Quart. J. Econ. cviii: 1-32.

1.1 Mathematical Modeling1-12V of the asset is given by a linear equation in time t, say, V mt b.The slope m must be negative since the value of the asset decreasesover time. The y-intercept is the initial value of the item and theslope gives the rate of depreciation (how much the item decreases invalue per time period).Example 7Straight-Line Depreciation A company has purchased a newgrinding machine for 100,000 with a useful life of 10 years, afterwhich it is assumed that the scrap value of the machine is 5000.Use straight-line depreciation to write an equation for the value Vof the machine where t is measured in years. What will be the valueof the machine after the first year? After the second year? After theninth year? What is the rate of depreciation?Solution:We assume that V mt b, where m is the slope and b is the V intercept. We then must find both m and b. We are told that themachine is initially worth 100,000, that is, when t 0, V 100, 000.Thus, the point (0, 100, 000) is on the line, and 100,000 is the V intercept, b. (see Figure 1.1.10). Note the domain of t is 0 t 10.Since the value of the machine in 10 years will be 5000, thismeans that when t 10, V 5000. Thus, (10, 5000) is also on theline. From Figure 1.1.10, the slope can then be calculated since wenow know that the two points (0, 100, 000) and (10, 5000) are on theline. Then5000 100, 000m 950010 0Figure 1.1.10The rate of depreciation is therefor -9500 /year. Then, using thepoint-slope form of a line,V 9500t 100, 000Where the time t is in years since the machine was purchased and Vis the value in dollars. Now we can find the value at different timeperiods,V (1) 9500(1) 100, 000 90, 500 or 90,500.V (2) 9500(2) 100, 000 81, 000 or 81,000.V (9) 9500(9) 100, 000 14, 500 or 14,500.T Technology Option. You may wish to see Technology Note 2 forthe solution to this example using the graphing calculator.⌈Technology Corner⌋Technology Note 1 - Example 1 on a Graphing Calculator. Begin byScreen 1.1.1Screen 1.1.2pressing the Y button on the top row of your calculator. Enter12 from the keypad and the variable X using the X,T,θ,n button.Next choose the viewing window by pressing the WINDOW buttonalong the top row of buttons. Since the smallest value for x is 0

1.1 Mathematical Modeling1-13(no steak specials sold), enter 0 for Xmin. We want to evaluate thefunction for 12 steak specials, or x 12, so choose an Xmax that isgreater than 12. The graph below used Xmin 20 and Xscl 5 (thatis, a tick mark is placed every 5 units). The range of values for ymust be large enough to view the function. The window below wasYmin 0, Ymax 200 and Yscl 10. The Xres setting can be left at 1 aswe want the full resolution on the screen. Press the GRAPH buttonto see the function displayed.To find the value of our function at a particular x-value, choosethe CALC menu (above the TRACE button) as shown in Screen 1.1.2.The trace function should avoided as it will not go to an exact xvalue. Choose the first option, 1:value and then enter the value 10.Pressing enter again to evaluate, we see in Screen 1.1.3 the value ofthe function at x 10 is 120.Technology Note 2 - Example 7 on a Graphing CalculatorThe depreciation function can be graphed as done in the Technology Note 1 above. Screen 1.1.4 shows the result of graphingY1 -9500X 100000 and finding the value at X 2. The window waschosen by entering Xmin 0 and Xmax 10, the known domain of thisfunction, and then pressing ZOOM and scrolling down to 0:ZoomFitand enter. This useful feature will evaluate the functions to begraphed from Xmin to Xmax and choose the values for Ymin and Ymaxto allow the functions to be seen.We were asked to find the value of the grinding machine at severaldifferent times, the table function can be used to simplify this task.Once a function is entered, go to the TBLSET feature by pressing2ND and then WINDOW , see Screen 1.1.5. We want to start atX 0 and count by 1’s, so set TblStart 0 and Tbl 1. To see thetable, press 2ND and then GRAPH , see Screen 1.1.6.Screen 1.1.4Screen 1.1.5Screen 1.1.61.1 SELF HELP EXERCISES1. Rogers and Akridge of Purdue Universitystudied fertilizer plants in Indiana. For a typical medium-sized plant they estimated fixedcosts at 400,000 and estimated the cost of eachton of fertilizer was 200 to produce. The plantsells its fertilizer output at 250 per ton.(a) Find and graph the cost, revenue, andprofit equations.(b) Determine the cost, revenue, and profits when the number of tons produced and soldis 5000, 7000, and 9000 tons.

1.1 Mathematical Modeling2. The excess supply and demand curves forwheat worldwide were estimated by Schmitzand coworkers to beSupply: p 7x 400Demand: p 510 3.5xwhere p is price per metric ton and x is inmillions of metric tons. Excess demand refers1-14to the excess of wheat that producer countrieshave over their own consumption. Graph thesetwo functions. Find the prices for the supplyand demand models when x is 70 million metrictons. Is the price for supply or demand larger?Repeat these questions when x is 100 millionmetric tons.EXERCISE SET 1.1In Exercises 1 and 2 you are given the cost peritem and the fixed costs. Assuming a linearcost model, find the cost equation, where C iscost and x is the number produced.1. Cost per item 3, fixed cost 10, 0002. Cost per item 6, fixed cost 14, 000In Exercises 3 and 4 you are given the priceof each item, which is assumed to be constant.Find the revenue equation, where R is revenueand x is the number sold.lowered by 5, then 20 additional items will besold.10. A company finds that at a price of 200,a total of 30 items will be sold. If the price israised 50, then 10 fewer items will be sold.In Exercises 11 to 14, find the supply equationusing the given information.11. A supplier will supply 50 items to the market if the price is 95 per item and supply 100items if the price is 175 per item.3. Price per item 5 4.Price per item 0.15. Using the cost equation found in Exercise 1and the revenue equation found in Exercise 3,find the profit equation for P , assuming thatthe number produced equals the number sold.6. Using the cost equation found in Exercise 2and the revenue equation found in Exercise 4,find the profit equation for P , assuming thatthe number produced equals the number sold.In questions 7 to 10, find the demand equationusing the given information.7. A company finds it can sell 10 items at aprice of 8.00 each and sell 15 items at a priceof 6.00 each.12. A supplier will supply 1000 items to themarket if the price is 3.00 per item and supply 2000 items if the price is 4.00 per item.13. At a price of 60 per item, a supplier willsupply 10 of these items. If the price increasesby 20, then 4 additional items will be supplied.14. At a price of 800 per item, a supplier willsupply 90 items. If the price decreases by 50,then the supplier will supply 20 fewer items.In Exercises 15 to 18, find the depreciationequation and corresponding domain using thegiven information.15. A calculator is purchased for 130 and thevalue decreases by 15 per year for 7 years.8. A company finds it can sell 40 items at aprice of 60.00 each and sell 60 items at a priceof 50.00 each.16. A violin bow is purchased for 50 and thevalue decreases by 5 per year for 6 years.9. A company finds that at a price of 35, atotal of 100 items will be sold. If the price is17. A car is purchased for 15,000 and is soldfor 6000 six years later.

1.1 Mathematical Modeling18. A car is purchased for 32,000 and is soldfor 23,200 eight years later.APPLICATIONS19. Revenue for red wine grapes in NapaValley. Brown and colleagues report that theprice of red varieties of grapes in Napa Valleywas 2274 per ton. Determine a revenue function and clearly indicate the independent anddependent varia

mx b a linear function. Definition of Linear Function A linear function f is any function of the form y f(x) mx b where m and b are constants. Example 2 Linear Functions Which of the following functions are linear? a. y 0.5x 12 b. 5y 2x 10 c. y 1/x 2 d. y x2 Solution: a. This is a linear function. The slope is m 0.5 and .

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