Systems of Equations and Inequalities

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1 7 Sstems of Equations and Inequalities CHAPTER OUTLINE Introduction Figure Enigma machines like this one, once owned b Italian dictator Benito Mussolini, were used b government and militar officials for enciphering and deciphering top-secret communications during World War II. (credit: Dave Adde, Flickr) 7. Sstems of Linear Equations: Two Variables 7. Sstems of Linear Equations: Three Variables 7. Sstems of Nonlinear Equations and Inequalities: Two Variables 7. Partial Fractions 7.5 Matrices and Matrix Operations 7.6 Solving Sstems with Gaussian Elimination 7.7 Solving Sstems with Inverses 7.8 Solving Sstems with Cramer's Rule B 9, it was obvious to the Nazi regime that defeat was imminent unless it could build a weapon with unlimited destructive power, one that had never been seen before in the histor of the world. In September, Adolf Hitler ordered German scientists to begin building an atomic bomb. Rumors and whispers began to spread from across the ocean. Refugees and diplomats told of the experiments happening in Norwa. However, Franklin D. Roosevelt wasn t sold, and even doubted British Prime Minister Winston Churchill s warning. Roosevelt wanted undeniable proof. Fortunatel, he soon received the proof he wanted when a group of mathematicians cracked the Enigma code, proving beond a doubt that Hitler was building an atomic bomb. The next da, Roosevelt gave the order that the United States begin work on the same. The Enigma is perhaps the most famous crptographic device ever known. It stands as an example of the pivotal role crptograph has plaed in societ. Now, technolog has moved crptanalsis to the digital world. Man ciphers are designed using invertible matrices as the method of message transference, as finding the inverse of a matrix is generall part of the process of decoding. In addition to knowing the matrix and its inverse, the receiver must also know the ke that, when used with the matrix inverse, will allow the message to be read. In this chapter, we will investigate matrices and their inverses, and various was to use matrices to solve sstems of equations. First, however, we will stud sstems of equations on their own: linear and nonlinear, and then partial fractions. We will not be breaking an secret codes here, but we will la the foundation for future courses. Download for free at 575

2 576 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES LEARNING OBJECTIVES In this section, ou will: Solve sstems of equations b graphing. Solve sstems of equations b substitution. Solve sstems of equations b addition. Identif inconsistent sstems of equations containing two variables. Express the solution of a sstem of dependent equations containing two variables. 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES Figure (credit: Thomas Sørenes) A skateboard manufacturer introduces a new line of boards. The manufacturer tracks its costs, which is the amount it spends to produce the boards, and its revenue, which is the amount it earns through sales of its boards. How can the compan determine if it is making a profit with its new line? How man skateboards must be produced and sold before a profit is possible? In this section, we will consider linear equations with two variables to answer these and similar questions. Introduction to Sstems of Equations In order to investigate situations such as that of the skateboard manufacturer, we need to recognize that we are dealing with more than one variable and likel more than one equation. A sstem of linear equations consists of two or more linear equations made up of two or more variables such that all equations in the sstem are considered simultaneousl. To find the unique solution to a sstem of linear equations, we must find a numerical value for each variable in the sstem that will satisf all equations in the sstem at the same time. Some linear sstems ma not have a solution and others ma have an infinite number of solutions. In order for a linear sstem to have a unique solution, there must be at least as man equations as there are variables. Even so, this does not guarantee a unique solution. In this section, we will look at sstems of linear equations in two variables, which consist of two equations that contain two different variables. For example, consider the following sstem of linear equations in two variables. x + = 5 x = 5 The solution to a sstem of linear equations in two variables is an ordered pair that satisfies each equation independentl. In this example, the ordered pair (, 7) is the solution to the sstem of linear equations. We can verif the solution b substituting the values into each equation to see if the ordered pair satisfies both equations. Shortl we will investigate methods of finding such a solution if it exists. () + (7) = 5 True () (7) = 5 True In addition to considering the number of equations and variables, we can categorize sstems of linear equations b the number of solutions. A consistent sstem of equations has at least one solution. A consistent sstem is considered to be an independent sstem if it has a single solution, such as the example we just explored. The two lines have Download for free at

3 SECTION 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES 577 different slopes and intersect at one point in the plane. A consistent sstem is considered to be a dependent sstem if the equations have the same slope and the same -intercepts. In other words, the lines coincide so the equations represent the same line. Ever point on the line represents a coordinate pair that satisfies the sstem. Thus, there are an infinite number of solutions. Another tpe of sstem of linear equations is an inconsistent sstem, which is one in which the equations represent two parallel lines. The lines have the same slope and different -intercepts. There are no points common to both lines; hence, there is no solution to the sstem. tpes of linear sstems There are three tpes of sstems of linear equations in two variables, and three tpes of solutions. An independent sstem has exactl one solution pair (x, ). The point where the two lines intersect is the onl solution. An inconsistent sstem has no solution. Notice that the two lines are parallel and will never intersect. A dependent sstem has infinitel man solutions. The lines are coincident. The are the same line, so ever coordinate pair on the line is a solution to both equations. Figure compares graphical representations of each tpe of sstem. 7, (, ) (, ) 5 6 x Independent Sstem Inconsistent Sstem Dependent Sstem Figure How To Given a sstem of linear equations and an ordered pair, determine whether the ordered pair is a solution.. Substitute the ordered pair into each equation in the sstem.. Determine whether true statements result from the substitution in both equations; if so, the ordered pair is a solution. Example Determining Whether an Ordered Pair Is a Solution to a Sstem of Equations Determine whether the ordered pair (5, ) is a solution to the given sstem of equations. x + = 8 x 9 = Solution Substitute the ordered pair (5, ) into both equations. (5) + () = 8 8 = 8 True (5) 9 = () = True The ordered pair (5, ) satisfies both equations, so it is the solution to the sstem. Analsis We can see the solution clearl b plotting the graph of each equation. Since the solution is an ordered pair that satisfies both equations, it is a point on both of the lines and thus the point of intersection of the two lines. See Figure. Download for free at

4 578 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES 5 (5, ) x + = x 9 = 5 x Figure Tr It # Determine whether the ordered pair (8, 5) is a solution to the following sstem. 5x = 0 x + = Solving Sstems of Equations b Graphing There are multiple methods of solving sstems of linear equations. For a sstem of linear equations in two variables, we can determine both the tpe of sstem and the solution b graphing the sstem of equations on the same set of axes. Example Solving a Sstem of Equations in Two Variables b Graphing Solve the following sstem of equations b graphing. Identif the tpe of sstem. x + = 8 x = Solution Solve the first equation for. Solve the second equation for. x + = 8 = x 8 x = = x + Graph both equations on the same set of axes as in Figure = x 8 = x + x (, ) 8 0 Figure Download for free at

5 SECTION 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES 579 The lines appear to intersect at the point (, ). We can check to make sure that this is the solution to the sstem b substituting the ordered pair into both equations. ( ) + ( ) = 8 8 = 8 True ( ) ( ) = = True The solution to the sstem is the ordered pair (, ), so the sstem is independent. Tr It # Solve the following sstem of equations b graphing. x 5 = 5 x + 5 = 5 Q & A Can graphing be used if the sstem is inconsistent or dependent? Yes, in both cases we can still graph the sstem to determine the tpe of sstem and solution. If the two lines are parallel, the sstem has no solution and is inconsistent. If the two lines are identical, the sstem has infinite solutions and is a dependent sstem. Solving Sstems of Equations b Substitution Solving a linear sstem in two variables b graphing works well when the solution consists of integer values, but if our solution contains decimals or fractions, it is not the most precise method. We will consider two more methods of solving a sstem of linear equations that are more precise than graphing. One such method is solving a sstem of equations b the substitution method, in which we solve one of the equations for one variable and then substitute the result into the second equation to solve for the second variable. Recall that we can solve for onl one variable at a time, which is the reason the substitution method is both valuable and practical. How To Given a sstem of two equations in two variables, solve using the substitution method.. Solve one of the two equations for one of the variables in terms of the other.. Substitute the expression for this variable into the second equation, then solve for the remaining variable.. Substitute that solution into either of the original equations to find the value of the first variable. If possible, write the solution as an ordered pair.. Check the solution in both equations. Example Solving a Sstem of Equations in Two Variables b Substitution Solve the following sstem of equations b substitution. x + = 5 x 5 = Solution First, we will solve the first equation for. x + = 5 = x 5 Download for free at

6 580 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES Now we can substitute the expression x 5 for in the second equation. x 5 = x 5(x 5) = x 5x + 5 = x = x = 8 Now, we substitute x = 8 into the first equation and solve for. (8) + = 5 = Our solution is (8, ). Check the solution b substituting (8, ) into both equations. x + = 5 (8) + () = 5 True x 5 = (8) 5() = True Tr It # Solve the following sstem of equations b substitution. x = + = x Q & A Can the substitution method be used to solve an linear sstem in two variables? Yes, but the method works best if one of the equations contains a coefficient of or so that we do not have to deal with fractions. Solving Sstems of Equations in Two Variables b the Addition Method A third method of solving sstems of linear equations is the addition method. In this method, we add two terms with the same variable, but opposite coefficients, so that the sum is zero. Of course, not all sstems are set up with the two terms of one variable having opposite coefficients. Often we must adjust one or both of the equations b multiplication so that one variable will be eliminated b addition. How To Given a sstem of equations, solve using the addition method.. Write both equations with x- and -variables on the left side of the equal sign and constants on the right.. Write one equation above the other, lining up corresponding variables. If one of the variables in the top equation has the opposite coefficient of the same variable in the bottom equation, add the equations together, eliminating one variable. If not, use multiplication b a nonzero number so that one of the variables in the top equation has the opposite coefficient of the same variable in the bottom equation, then add the equations to eliminate the variable.. Solve the resulting equation for the remaining variable.. Substitute that value into one of the original equations and solve for the second variable. 5. Check the solution b substituting the values into the other equation. Download for free at

7 SECTION 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES 58 Example Solving a Sstem b the Addition Method Solve the given sstem of equations b addition. x + = x + = Solution Both equations are alread set equal to a constant. Notice that the coefficient of x in the second equation,, is the opposite of the coefficient of x in the first equation,. We can add the two equations to eliminate x without needing to multipl b a constant. x + = x + = = Now that we have eliminated x, we can solve the resulting equation for. = = _ Then, we substitute this value for into one of the original equations and solve for x. x + = x + = x = _ x = 7 _ The solution to this sstem is ( 7, ) Check the solution in the first equation. x = 7 _ x + = ( 7 ) + ( ) = 7 + = = = True Analsis We gain an important perspective on sstems of equations b looking at the graphical representation. See Figure 5 to find that the equations intersect at the solution. We do not need to ask whether there ma be a second solution because observing the graph confirms that the sstem has exactl one solution. x + = x + = 7, x Figure 5 Download for free at

8 58 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES Example 5 Using the Addition Method When Multiplication of One Equation Is Required Solve the given sstem of equations b the addition method. x + 5 = x = Solution Adding these equations as presented will not eliminate a variable. However, we see that the first equation has x in it and the second equation has x. So if we multipl the second equation b, the x-terms will add to zero. Now, let s add them. x = (x ) = () Multipl both sides b. x + 6 = x + 5 = x + 6 = = = For the last step, we substitute = into one of the original equations and solve for x. x + 5 = x + 5( ) = x 0 = x = 9 x = Use the distributive propert. Our solution is the ordered pair (, ). See Figure 6. Check the solution in the original second equation. x = () ( ) = + 8 = True x 5 6 (, ) x = x + 5 = Figure 6 Tr It # Solve the sstem of equations b addition. x 7 = x + = 0 Example 6 Using the Addition Method When Multiplication of Both Equations Is Required Solve the given sstem of equations in two variables b addition. x + = 6 5x 0 = 0 Download for free at

9 SECTION 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES 58 Solution One equation has x and the other has 5x. The least common multiple is 0x so we will have to multipl both equations b a constant in order to eliminate one variable. Let s eliminate x b multipling the first equation b 5 and the second equation b. 5(x + ) = 5( 6) Then, we add the two equations together. Substitute = into the original first equation. 0x 5 = 80 (5x 0) = (0) 0x 0 = 60 0x 5 = 80 0x 0 = 60 5 = 0 = x + ( ) = 6 The solution is (, ). Check it in the other equation. See Figure 7. x = 6 x = x = 5x 0 = 0 5( ) 0( ) = = 0 0 = x + = 6 (, ) 5 5x 0 = x Example 7 Figure 7 Using the Addition Method in Sstems of Equations Containing Fractions Solve the given sstem of equations in two variables b addition. x + 6 = x = Solution First clear each equation of fractions b multipling both sides of the equation b the least common denominator. 6 ( x + 6) = 6() x + = 8 ( x ) = () x = Download for free at

10 58 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES Now multipl the second equation b so that we can eliminate the x-variable. (x ) = () x + = Add the two equations to eliminate the x-variable and solve the resulting equation. Substitute = 7 into the first equation. The solution is (, 7 ). Check it in the other equation. Tr It #5 Solve the sstem of equations b addition. x + = 8 x + = = = 7 x + (7) = 8 x x = x = = 5.5 = 7 = 7 = = x + = 8 x + 5 = 0 Identifing Inconsistent Sstems of Equations Containing Two Variables Now that we have several methods for solving sstems of equations, we can use the methods to identif inconsistent sstems. Recall that an inconsistent sstem consists of parallel lines that have the same slope but different -intercepts. The will never intersect. When searching for a solution to an inconsistent sstem, we will come up with a false statement, such as = 0. Example 8 Solving an Inconsistent Sstem of Equations Solve the following sstem of equations. x = 9 x + = Solution We can approach this problem in two was. Because one equation is alread solved for x, the most obvious step is to use substitution. x + = (9 ) + = = 9 = Clearl, this statement is a contradiction because 9. Therefore, the sstem has no solution. Download for free at

11 SECTION 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES 585 The second approach would be to first manipulate the equations so that the are both in slope-intercept form. We manipulate the first equation as follows. x = 9 = x + 9 = x + 9 We then convert the second equation expressed to slope-intercept form. x + = = x + = x + Comparing the equations, we see that the have the same slope but different -intercepts. Therefore, the lines are parallel and do not intersect. = x + 9 = x + Analsis Writing the equations in slope-intercept form confirms that the sstem is inconsistent because all lines will intersect eventuall unless the are parallel. Parallel lines will never intersect; thus, the two lines have no points in common. The graphs of the equations in this example are shown in Figure 8. Tr It #6 = x Figure 8 Solve the following sstem of equations in two variables. x = x = 6 = x + x Expressing the Solution of a Sstem of Dependent Equations Containing Two Variables Recall that a dependent sstem of equations in two variables is a sstem in which the two equations represent the same line. Dependent sstems have an infinite number of solutions because all of the points on one line are also on the other line. After using substitution or addition, the resulting equation will be an identit, such as 0 = 0. Example 9 Finding a Solution to a Dependent Sstem of Linear Equations Find a solution to the sstem of equations using the addition method. x + = x + 9 = 6 Solution With the addition method, we want to eliminate one of the variables b adding the equations. In this case, let s focus on eliminating x. If we multipl both sides of the first equation b, then we will be able to eliminate the x-variable. Download for free at

12 586 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES Now add the equations. x + = ( )(x + ) = ( )() x 9 = 6 x 9 = 6 + x + 9 = 6 0 = 0 We can see that there will be an infinite number of solutions that satisf both equations. Analsis If we rewrote both equations in the slope-intercept form, we might know what the solution would look like before adding. Let s look at what happens when we convert the sstem to slope-intercept form. x + = = x + = x + x + 9 = 6 9 = x + 6 = 9 x = x + See Figure 9. Notice the results are the same. The general solution to the sstem is ( x, _ x + _ ). 5 x + = 5 5 x 5 x + 9 = 6 Figure 9 Tr It #7 Solve the following sstem of equations in two variables. x = 5 + 6x = 5 Using Sstems of Equations to Investigate Profits Using what we have learned about sstems of equations, we can return to the skateboard manufacturing problem at the beginning of the section. The skateboard manufacturer s revenue function is the function used to calculate the amount of mone that comes into the business. It can be represented b the equation R = xp, where x = quantit and p = price. The revenue function is shown in orange in Figure 0. The cost function is the function used to calculate the costs of doing business. It includes fixed costs, such as rent and salaries, and variable costs, such as utilities. The cost function is shown in blue in Figure 0. The x-axis represents quantit in hundreds of units. The -axis represents either cost or revenue in hundreds of dollars. Download for free at

13 SECTION 7. SYSTEMS OF LINEAR EQUATIONS: TWO VARIABLES 587 Mone (in hundreds of dollars) Cost Revenue (7, ) Break-even Profit Quantit (in hundreds of units) Figure 0 The point at which the two lines intersect is called the break-even point. We can see from the graph that if 700 units are produced, the cost is $,00 and the revenue is also $,00. In other words, the compan breaks even if the produce and sell 700 units. The neither make mone nor lose mone. The shaded region to the right of the break-even point represents quantities for which the compan makes a profit. The shaded region to the left represents quantities for which the compan suffers a loss. The profit function is the revenue function minus the cost function, written as P(x) = R(x) C(x). Clearl, knowing the quantit for which the cost equals the revenue is of great importance to businesses. Example 0 Finding the Break-Even Point and the Profit Function Using Substitution Given the cost function C(x) = 0.85x + 5,000 and the revenue function R(x) =.55x, find the break-even point and the profit function. Solution Write the sstem of equations using to replace function notation. = 0.85x + 5,000 =.55x Substitute the expression 0.85x + 5,000 from the first equation into the second equation and solve for x. 0.85x + 5,000 =.55x 5,000 = 0.7x 50,000 = x Then, we substitute x = 50,000 into either the cost function or the revenue function. The break-even point is (50,000, 77,500)..55(50,000) = 77,500 The profit function is found using the formula P(x) = R(x) C(x). The profit function is P(x) = 0.7x 5,000. P(x) =.55x (0.85x + 5, 000) = 0.7x 5, 000 Analsis The cost to produce 50,000 units is $77,500, and the revenue from the sales of 50,000 units is also $77,500. To make a profit, the business must produce and sell more than 50,000 units. See Figure. We see from the graph in Figure that the profit function has a negative value until x = 50,000, when the graph crosses the x-axis. Then, the graph emerges into positive -values and continues on this path as the profit function is a straight line. This illustrates that the break-even point for businesses occurs when the profit function is 0. The area to the left of the break-even point represents operating at a loss. Download for free at

14 588 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES Dollars 00,000 80,000 60,000 0,000 0,000 0 Example 0 0,000 0,000 60,000 80,000 00,000 Quantit Profit Break-even point (50,000, 77,500) Cost C(x) = 0.85x + 5,000 Revenue R(x) =.55x Figure Dollars profit 60,000 50,000 0,000 0,000 0,000 0, ,000 0,000 0,000 0,000 0,000 0,000 Writing and Solving a Sstem of Equations in Two Variables 0 0,000 0,000 0,000 0,000 Profit Profit P(x) = 0.7x 5,000 Figure Quantit Break-even point (50,000, 0) 50,000 60,000 70,000 80,000 90,000 00,000 0,000 0,000 The cost of a ticket to the circus is $5.00 for children and $50.00 for adults. On a certain da, attendance at the circus is,000 and the total gate revenue is $70,000. How man children and how man adults bought tickets? Solution Let c = the number of children and a = the number of adults in attendance. The total number of people is,000. We can use this to write an equation for the number of people at the circus that da. c + a =,000 The revenue from all children can be found b multipling $5.00 b the number of children, 5c. The revenue from all adults can be found b multipling $50.00 b the number of adults, 50a. The total revenue is $70,000. We can use this to write an equation for the revenue. 5c + 50a = 70,000 We now have a sstem of linear equations in two variables. c + a =,000 5c + 50a = 70,000 In the first equation, the coefficient of both variables is. We can quickl solve the first equation for either c or a. We will solve for a. c + a =,000 a =,000 c Substitute the expression,000 c in the second equation for a and solve for c. 5c + 50(,000 c) = 70,000 5c + 00,000 50c = 70,000 Substitute c =,00 into the first equation to solve for a. 5c = 0,000 c =,00,00 + a =,000 a = 800 We find that,00 children and 800 adults bought tickets to the circus that da. Tr It #8 Meal tickets at the circus cost $.00 for children and $.00 for adults. If,650 meal tickets were bought for a total of $,00, how man children and how man adults bought meal tickets? Access these online resources for additional instruction and practice with sstems of linear equations. Solving Sstems of Equations Using Substitution ( Solving Sstems of Equations Using Elimination ( Applications of Sstems of Equations ( Download for free at

15 SECTION 7. SECTION EXERCISES SECTION EXERCISES VERBAL. Can a sstem of linear equations have exactl two solutions? Explain wh or wh not.. If ou are solving a break-even analsis and get a negative break-even point, explain what this signifies for the compan? 5. Given a sstem of equations, explain at least two different methods of solving that sstem.. If ou are performing a break-even analsis for a business and their cost and revenue equations are dependent, explain what this means for the compan s profit margins.. If ou are solving a break-even analsis and there is no break-even point, explain what this means for the compan. How should the ensure there is a break-even point? ALGEBRAIC For the following exercises, determine whether the given ordered pair is a solution to the sstem of equations. 6. 5x = x + 6 = and (, 0) 9. x + 5 = 7 x + 9 = 7 and (, ) 7. x 5 = x + = 0 and ( 6, ) 0. x + 8 = x = and (, 5) For the following exercises, solve each sstem b substitution. 8. x + 7 = x + = 0 and (, ). x + = 5 x + =. x = 8 5x + 0 = 0. x + = 0 x + 9 = 0. x + =.8 9x 5 =. 5. x + =. x 6 =.8 6. x 0. = 0x + = 5 7. x + 5 = 9 0x + 50 = x + = x = 8 9. x + = 6 0. x + = 6 x + = 9 8 x + = For the following exercises, solve each sstem b addition.. x + 5 = 7x + = 0. 6x 5 = x + 6 =. 5x =.6 x 6 =.. 7x = x + 5 =.5 5. x + = 5x 0 = x + 0. = 0.6 x = 6. 7x + 6 = 8x = x + 0. = 0.6 5x 0 = x + = 0 8 x = 0 8. x + 9 = 9 x + 5 = For the following exercises, solve each sstem b an method.. 5x + 9 = 6 x + =. 6x 8 = 0.6 x + = x =.5 7x =. x 5 = 55 6x + 5 = 55 Download for free at

16 590 CHAPTER 7 SYSTEMS OF EQUATIONS AND INEQUALITIES 5. 7x = 7 6 x + = 6. x + 6 = x + = x 6 = 6 x + = 8. x + = x + = 8 9..x +. = 0..x +. = x + 0. = 0.5x 0. = 0 GRAPHICAL For the following exercises, graph the sstem of equations and state whether the sstem is consistent, inconsistent, or dependent and whether the sstem has one solution, no solution, or infinite solutions.. x = 0.6 x =.. x 5 = 7 x = TECHNOLOGY. x + = x = 5. x = 5 9x + 6 = 5. x + = 7 x + 6 = For the following exercises, use the intersect function on a graphing device to solve each sstem. Round all answers to the nearest hundredth x + 0. = 0. 0.x = x + 0. = x = x + 0. = 0.5x 0.9 = x = x = x = x =. EXTENSIONS For the following exercises, solve each sstem in terms of A, B, C, D, E, and F where A F are nonzero numbers. Note that A B and AE BD. 5. x + = A x = B 5. x + A = x + B = 5. Ax + = 0 Bx + = 5. Ax + B = C x + = 55. Ax + B = C Dx + E = F REAL-WORLD APPLICATIONS For the following exercises, solve for the desired quantit. 56. A stuffed animal business has a total cost of production C = x + 0 and a revenue function R = 0x. Find the break-even point. 58. A cell phone factor has a cost of production C(x) = 50x + 0,000 and a revenue function R(x) = 00x. What is the break-even point? 60. A guitar factor has a cost of production C(x) = 75x + 50,000. If the compan needs to break even after 50 units sold, at what price should the sell each guitar? Round up to the nearest dollar, and write the revenue function. 57. A fast-food restaurant has a cost of production C(x) = x + 0 and a revenue function R(x) = 5x. When does the compan start to turn a profit? 59. A musician charges C(x) = 6x + 0,000, where x is the total number of attendees at the concert. The venue charges $80 per ticket. After how man people bu tickets does the venue break even, and what is the value of the total tickets sold at that point? Download for free at

17 SECTION 7. SECTION EXERCISES 59 For the following exercises, use a sstem of linear equations with two variables and two equations to solve. 6. Find two numbers whose sum is 8 and difference is. 6. The startup cost for a restaurant is $0,000, and each meal costs $0 for the restaurant to make. If each meal is then sold for $5, after how man meals does the restaurant break even? 65. A total of,595 first- and second-ear college students gathered at a pep rall. The number of freshmen exceeded the number of sophomores b 5. How man freshmen and sophomores were in attendance? 67. There were 0 facult at a conference. If there were 8 more women than men attending, how man of each gender attended the conference? 69. If a scientist mixed 0% saline solution with 60% saline solution to get 5 gallons of 0% saline solution, how man gallons of 0% and 60% solutions were mixed? 7. An investor who dabbles in real estate invested. million dollars into two land investments. On the first investment, Swan Peak, her return was a 0% increase on the mone she invested. On the second investment, Riverside Communit, she earned 50% over what she invested. If she earned $ million in profits, how much did she invest in each of the land deals? 7. If an investor invests $,000 into two bonds, one that pas % in simple interest, and the other paing % simple interest, and the investor earns $70.00 annual interest, how much was invested in each account? 75. A store clerk sold 60 pairs of sneakers. The high-tops sold for $98.99 and the low-tops sold for $9.99. If the receipts for the two tpes of sales totaled $6,0.0, how man of each tpe of sneaker were sold? 77. Admission into an amusement park for children and adults is $6.90. For 6 children and adults, the admission is $75.5. Assuming a different price for children and adults, what is the price of the child s ticket and the price of the adult ticket? 6. A number is 9 more than another number. Twice the sum of the two numbers is 0. Find the two numbers. 6. A moving compan charges a flat rate of $50, and an additional $5 for each box. If a taxi service would charge $0 for each box, how man boxes would ou need for it to be cheaper to use the moving compan, and what would be the total cost? students enrolled in a freshman-level chemistr class. B the end of the semester, 5 times the number of students passed as failed. Find the number of students who passed, and the number of students who failed. 68. A jeep and BMW enter a highwa running eastwest at the same exit heading in opposite directions. The jeep entered the highwa 0 minutes before the BMW did, and traveled 7 mph slower than the BMW. After hours from the time the BMW entered the highwa, the cars were 06.5 miles apart. Find the speed of each car, assuming the were driven on cruise control. 70. An investor earned triple the profits of what she earned last ear. If she made $500,000.8 total for both ears, how much did she earn in profits each ear? 7. If an investor invests a total of $5,000 into two bonds, one that pas % simple interest, and the other that pas 7 % interest, and the investor 8 earns $77.50 annual interest, how much was invested in each account? 7. CDs cost $5.96 more than DVDs at All Bets Are Off Electronics. How much would 6 CDs and DVDs cost if 5 CDs and DVDs cost $7.7? 76. A concert manager counted 50 ticket receipts the da after a concert. The price for a student ticket was $.50, and the price for an adult ticket was $6.00. The register confirms that $5,075 was taken in. How man student tickets and adult tickets were sold? Download for free at