A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 794 794 Chapter 8 Systems of Equations and Inequalities Section 8.5 Objectives � Graph a linear inequality in � � � two variables. Graph a nonlinear inequality in two variables. Use mathematical models involving linear inequalities. Graph a system of inequalities. Systems of Inequalities W e opened the chapter noting that the modern emphasis on thinness as the ideal body shape has been suggested as a major cause of eating disorders. In this section, as well as in the exercise set, we use systems of linear inequalities in two variables that will enable you to establish a healthy weight range for your height and age. Linear Inequalities in Two Variables and Their Solutions We have seen that equations in the form Ax + By = C are straight lines when graphed. If we change the symbol = to 7, 6, Ú, or …, we obtain a linear inequality in two variables. Some examples of linear inequalities in two variables are x + y 7 2, 3x - 5y … 15, and 2x - y 6 4. A solution of an inequality in two variables, x and y, is an ordered pair of real numbers with the following property: When the x-coordinate is substituted for x and the y-coordinate is substituted for y in the inequality, we obtain a true statement. For example, (3, 2) is a solution of the inequality x + y 7 1. When 3 is substituted for x and 2 is substituted for y, we obtain the true statement 3 + 2 7 1, or 5 7 1. Because there are infinitely many pairs of numbers that have a sum greater than 1, the inequality x + y 7 1 has infinitely many solutions. Each ordered-pair solution is said to satisfy the inequality. Thus, (3, 2) satisfies the inequality x + y 7 1. � Graph a linear inequality in two variables. The Graph of a Linear Inequality in Two Variables We know that the graph of an equation in two variables is the set of all points whose coordinates satisfy the equation. Similarly, the graph of an inequality in two variables is the set of all points whose coordinates satisfy the inequality. Let’s use Figure 8.14 to get an idea of what the y graph of a linear inequality in two variables looks 5 like. Part of the figure shows the graph of the linear 4 equation x + y = 2. The line divides the points in Half-plane 3 x+y>2 the rectangular coordinate system into three sets. 2 First, there is the set of points along the line, 1 x satisfying x + y = 2. Next, there is the set of points −5 −4 −3 −2 −1−1 1 2 3 4 5 in the green region above the line. Points in the −2 green region satisfy the linear inequality x + y 7 2. Half-plane −3 x + y < 2 Finally, there is the set of points in the purple region −4 Line x + y = 2 below the line. Points in the purple region satisfy the −5 linear inequality x + y 6 2. A half-plane is the set of all the points on one Figure 8.14 side of a line. In Figure 8.14, the green region is a half-plane. The purple region is also a half-plane. A half-plane is the graph of a linear inequality that involves 7 or 6. The graph of an inequality that involves Ú or … is a half-plane and a line. A solid line is used to show that a line is part of a graph. A dashed line is used to show that a line is not part of a graph. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 795 Section 8.5 Systems of Inequalities 795 Graphing a Linear Inequality in Two Variables 1. Replace the inequality symbol with an equal sign and graph the corresponding linear equation. Draw a solid line if the original inequality contains a … or Ú symbol. Draw a dashed line if the original inequality contains a 6 or 7 symbol. 2. Choose a test point from one of the half-planes. (Do not choose a point on the line.) Substitute the coordinates of the test point into the inequality. 3. If a true statement results, shade the half-plane containing this test point. If a false statement results, shade the half-plane not containing this test point. Graphing a Linear Inequality in Two Variables EXAMPLE 1 Graph: 2x - 3y Ú 6. Solution Step 1 Replace the inequality symbol by ⴝ and graph the linear equation. We need to graph 2x - 3y = 6. We can use intercepts to graph this line. y We set y ⴝ 0 to find the x-intercept. 5 4 3 2 1 1 2 3 4 5 6 7 8 −2 −3 −4 −5 2x - 3y = 6 3#0 2 # 0 - 3y = 6 -3y = 6 x = 6 2x = 6 x = 3 y = -2 2x − 3y = 6 The x-intercept is 3, so the line passes through (3, 0). The y-intercept is -2, so the line passes through 10, -22. Using the intercepts, the line is shown in Figure 8.15 as a solid line. This is because the inequality 2x - 3y Ú 6 contains a Ú symbol, in which equality is included. Step 2 Choose a test point from one of the half-planes and not from the line. Substitute its coordinates into the inequality. The line 2x - 3y = 6 divides the plane into three parts—the line itself and two half-planes. The points in one half-plane satisfy 2x - 3y 7 6. The points in the other half-plane satisfy 2x - 3y 6 6. We need to find which half-plane belongs to the solution of 2x - 3y Ú 6. To do so, we test a point from either half-plane. The origin, (0, 0), is the easiest point to test. Figure 8.15 Preparing to graph 2x - 3y Ú 6 y 2x - 3y Ú 6 5 4 3 2 1 −2 −1−1 2x - 3y = 6 2x - −2 −1−1 We set x ⴝ 0 to find the y-intercept. ? 2#0 - 3#0 Ú 6 ? 0 - 0 Ú 6 1 2 3 4 5 6 7 8 −2 −3 −4 −5 Figure 8.16 The graph of 2x - 3y Ú 6 0 Ú 6 x This is the given inequality. Test (0, 0) by substituting 0 for x and 0 for y. Multiply. This statement is false. Step 3 If a false statement results, shade the half-plane not containing the test point. Because 0 is not greater than or equal to 6, the test point, (0, 0), is not part of the solution set. Thus, the half-plane below the solid line 2x - 3y = 6 is part of the solution set. The solution set is the line and the half-plane that does not contain the point (0, 0), indicated by shading this half-plane. The graph is shown using green shading and a blue line in Figure 8.16. Check Point 1 Graph: 4x - 2y Ú 8. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 796 796 Chapter 8 Systems of Equations and Inequalities When graphing a linear inequality, choose a test point that lies in one of the halfplanes and not on the line dividing the half-planes. The test point (0, 0) is convenient because it is easy to calculate when 0 is substituted for each variable. However, if (0, 0) lies on the dividing line and not in a half-plane, a different test point must be selected. Graphing a Linear Inequality in Two Variables EXAMPLE 2 Graph: Solution y y= − 23 x 5 4 3 2 1 −5 −4 −3 −2 −1−1 Rise = −2 −2 −3 −4 −5 Step 1 Replace the inequality symbol by ⴝ and graph the linear equation. Because we are interested in graphing y 7 - 23 x, we begin by graphing y = - 23 x. We can use the slope and the y-intercept to graph this linear function. Test point: (1, 1) 1 2 3 4 5 y= – x Slope = Run = 3 Figure 8.17 The graph of y 7 - 23 x 2 y 7 - x. 3 2 x+0 3 −2 rise = 3 run y-intercept = 0 The y-intercept is 0, so the line passes through (0, 0). Using the y-intercept and the slope, the line is shown in Figure 8.17 as a dashed line. This is because the inequality y 7 - 23 x contains a 7 symbol, in which equality is not included. Step 2 Choose a test point from one of the half-planes and not from the line. Substitute its coordinates into the inequality. We cannot use (0, 0) as a test point because it lies on the line and not in a half-plane. Let’s use (1, 1), which lies in the half-plane above the line. 2 y 7 - x 3 This is the given inequality. 1 7 - 2# 1 3 Test (1, 1) by substituting 1 for x and 1 for y. 1 7 - 2 3 This statement is true. ? Step 3 If a true statement results, shade the half-plane containing the test point. Because 1 is greater than - 23 , the test point (1, 1) is part of the solution set. All the points on the same side of the line y = - 23 x as the point (1, 1) are members of the solution set. The solution set is the half-plane that contains the point (1, 1), indicated by shading this half-plane. The graph is shown using green shading and a dashed blue line in Figure 8.17. Technology Most graphing utilities can graph inequalities in two variables with the 冷 SHADE 冷 feature. The procedure varies by model, so consult your manual. For most graphing utilities, you must first solve for y if it is not already isolated. The figure shows the graph of y 7 - 23 x. Most displays do not distinguish between dashed and solid boundary lines. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 797 Section 8.5 Systems of Inequalities 2 Check Point Graph: 797 3 y 7 - x. 4 Graphing Linear Inequalities without Using Test Points You can graph inequalities in the form y 7 mx + b or y 6 mx + b without using test points. The inequality symbol indicates which half-plane to shade. • If y 7 mx + b, shade the half-plane above the line y = mx + b. • If y 6 mx + b, shade the half-plane below the line y = mx + b. Observe how this is illustrated in Figure 8.17 in the margin on the previous page. The graph of y 7 - 23 x is the half-plane above the line y = - 23 x. It is also not necessary to use test points when graphing inequalities involving half-planes on one side of a vertical or a horizontal line. For the Vertical Line x ⴝ a: For the Horizontal Line y ⴝ b: • If x 7 a, shade the half-plane to the right of x = a. • If y 7 b, shade the half-plane above y = b. • If x 6 a, shade the half-plane to the left of x = a. • If y 6 b, shade the half-plane below y = b. Study Tip Continue using test points to graph inequalities in the form Ax + By 7 C or Ax + By 6 C. The graph of Ax + By 7 C can lie above or below the line given by Ax + By = C, depending on the value of B. The same comment applies to the graph of Ax + By 6 C. x < a in the yellow region. x > a in the green region. y y y > b in the green region. y=b x x y < b in the yellow region. x=a EXAMPLE 3 Graphing Inequalities without Using Test Points Graph each inequality in a rectangular coordinate system: a. y … - 3 b. x 7 2. Solution b. x 7 2 a. y … - 3 Graph y = −3, a horizontal line with y-intercept −3. The line is solid because equality is included in y −3. Because of the less than part of , shade the half-plane below the horizontal line. y 5 4 3 2 1 −5 −4 −3 −2 −1−1 −2 −3 −4 −5 1 2 3 4 5 x Graph x = 2, a vertical line with x-intercept 2. The line is dashed because equality is not included in x > 2. Because of >, the greater than symbol, shade the half-plane to the right of the vertical line. y 5 4 3 2 1 −5 −4 −3 −2 −1−1 −2 −3 −4 −5 1 2 3 4 5 x A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 798 798 Chapter 8 Systems of Equations and Inequalities 3 Check Point Graph each inequality in a rectangular coordinate system: a. y 7 1 � Graph a nonlinear inequality in two variables. b. x … - 2. Graphing a Nonlinear Inequality in Two Variables Example 4 illustrates that a nonlinear inequality in two variables is graphed in the same way that we graph a linear inequality. Graphing a Nonlinear Inequality in Two Variables EXAMPLE 4 Graph: x2 + y2 … 9. Solution Step 1 Replace the inequality symbol with ⴝ and graph the nonlinear equation. We need to graph x2 + y2 = 9. The graph is a circle of radius 3 with its center at the origin. The graph is shown in Figure 8.18 as a solid circle because equality is included in the … symbol. Step 2 Choose a test point from one of the regions and not from the circle. Substitute its coordinates into the inequality. The circle divides the plane into three parts—the circle itself, the region inside the circle, and the region outside the circle. We need to determine whether the region inside or outside the circle is included in the solution. To do so, we will use the test point (0, 0) from inside the circle. x2 + y2 … 9 This is the given inequality. ? 02 + 02 … 9 Test (0, 0) by substituting 0 for x and 0 for y. ? Square 0: 02 = 0. 0 + 0 … 9 0 … 9 Add. This statement is true. Step 3 If a true statement results, shade the region containing the test point. The true statement tells us that all the points inside the circle satisfy x2 + y2 … 9. The graph is shown using green shading and a solid blue circle in Figure 8.19. y y 5 4 3 2 1 −5 −4 −3 −2 −1−1 5 4 3 2 1 1 2 3 4 5 x −2 −3 −4 −5 −5 −4 −3 −2 −1−1 1 2 3 4 5 −2 −3 −4 −5 Figure 8.18 Preparing to graph Figure 8.19 The graph of x2 + y2 … 9 x2 + y2 … 9 Check Point 4 Graph: x 2 + y2 Ú 16. x A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 799 Section 8.5 Systems of Inequalities Use mathematical models involving linear inequalities. Modeling with Systems of Linear Inequalities Just as two or more linear equations make up a system of linear equations, two or more linear inequalities make up a system of linear inequalities. A solution of a system of linear inequalities in two variables is an ordered pair that satisfies each inequality in the system. Does Your Weight Fit You? EXAMPLE 5 The latest guidelines, which apply to both men and women, give healthy weight ranges, rather than specific weights, for your height. Figure 8.20 shows the healthy weight region for various heights for people between the ages of 19 and 34, inclusive. Healthy Weight Region for Men and Women, Ages 19 to 34 y 230 210 Weight (pounds) � 799 4.9x − y = 165 190 A 170 Healthy Weight Region 150 130 B 3.7x − y = 125 110 90 x 60 62 64 66 68 70 72 Height (inches) 74 76 78 Figure 8.20 Source: U.S. Department of Health and Human Services If x represents height, in inches, and y represents weight, in pounds, the healthy weight region in Figure 8.20 can be modeled by the following system of linear inequalities: b 4.9x - y Ú 165 3.7x - y … 125. Show that point A in Figure 8.20 is a solution of the system of inequalities that describes healthy weight. Solution Point A has coordinates (70, 170). This means that if a person is 70 inches tall, or 5 feet 10 inches, and weighs 170 pounds, then that person’s weight is within the healthy weight region. We can show that (70, 170) satisfies the system of inequalities by substituting 70 for x and 170 for y in each inequality in the system. 4.9x - y Ú 165 3.7x - y … 125 ? ? 4.91702 - 170 Ú 165 3.71702 - 170 … 125 ? 343 - 170 Ú 165 173 Ú 165, ? true 259 - 170 … 125 89 … 125, true The coordinates (70, 170) make each inequality true. Thus, (70, 170) satisfies the system for the healthy weight region and is a solution of the system. Check Point 5 Show that point B in Figure 8.20 is a solution of the system of inequalities that describes healthy weight. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 800 800 Chapter 8 Systems of Equations and Inequalities � Graphing Systems of Linear Inequalities Graph a system of inequalities. The solution set of a system of linear inequalities in two variables is the set of all ordered pairs that satisfy each inequality in the system. Thus, to graph a system of inequalities in two variables, begin by graphing each individual inequality in the same rectangular coordinate system. Then find the region, if there is one, that is common to every graph in the system. This region of intersection gives a picture of the system’s solution set. Graphing a System of Linear Inequalities EXAMPLE 6 Graph the solution set of the system: b x - y 6 1 2x + 3y Ú 12. Solution Replacing each inequality symbol with an equal sign indicates that we need to graph x - y = 1 and 2x + 3y = 12. We can use intercepts to graph these lines. xⴚyⴝ1 2x ⴙ 3y ⴝ 12 Set y = 0 in each equation. x-intercept: x-0=1 x=1 The line passes through (1, 0). Set x = 0 in y-intercept: 0-y=1 each equation. –y=1 y=–1 The line passes through (0, –1). 2x+3 ⴢ 0=12 2x=12 x=6 The line passes through (6, 0). x-intercept: 2 ⴢ 0+3y=12 3y=12 y=4 The line passes through (0, 4). y-intercept: Now we are ready to graph the solution set of the system of linear inequalities. Graph x − y < 1. The blue line, x − y = 1, is dashed: Equality is not included in x − y < 1. Because (0, 0) makes the inequality true (0 − 0 < 1, or 0 < 1, is true), shade the halfplane containing (0, 0) in yellow. Add the graph of 2x + 3y 12. The red line, 2x + 3y = 12, is solid: Equality is included in 2x + 3y 12. Because (0, 0) makes the inequality false (2 ⴢ 0 + 3 ⴢ 0 12, or 0 12, is false), shade the half-plane not containing (0, 0) using green vertical shading. y −2 −3 −4 −5 y y 5 4 3 2 1 −5 −4 −3 −2 −1−1 The solution set of the system is graphed as the intersection (the overlap) of the two half-planes. This is the region in which the yellow shading and the green vertical shading overlap. 2x + 3y = 12: passes through (6, 0) and (0, 4) 1 2 3 4 5 x x − y = 1: passes through (1, 0) and (0, −1) The graph of x-y<1 5 4 3 2 1 5 4 3 2 1 −5 −4 −3 −2 −1−1 −2 −3 −4 −5 1 2 3 4 5 x−y=1 Adding the graph of 2x+3y 12 x −5 −4 −3 −2 −1−1 −2 −3 −4 −5 1 2 3 4 5 x This open dot shows (3, 2) is not in the solution set. It does not satisfy x − y < 1. The graph of x-y<1 and 2x+3y 12 A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 801 Section 8.5 Systems of Inequalities Check Point 6 801 Graph the solution set of the system: b x - 3y 6 6 2x + 3y Ú - 6. Graphing a System of Inequalities EXAMPLE 7 Graph the solution set of the system: b y Ú x2 - 4 x - y Ú 2. Solution We begin by graphing y Ú x2 - 4. Because equality is included in Ú, we graph y = x2 - 4 as a solid parabola. Because (0, 0) makes the inequality y Ú x2 - 4 true (we obtain 0 Ú - 4), we shade the interior portion of the parabola containing (0, 0), shown in yellow in Figure 8.21. y 5 4 3 2 1 −5 −4 −3 −2 −1−1 y y y = x2 − 4 1 2 3 4 5 x −2 −3 −4 −5 Figure 8.21 The graph of y Ú x2 - 4 5 4 3 2 1 y = x2 − 4 −5 −4 −3 −2 −1−1 x−y=2 x 1 2 3 4 5 −2 −3 −4 −5 y = x2 − 4 5 4 3 2 1 −5 −4 −3 −2 −1−1 (−1, −3) −2 x−y=2 1 3 4 5 x (2, 0) −4 −5 Figure 8.22 Adding the graph of x - y Ú 2 Figure 8.23 The graph of y Ú x2 - 4 and x - y Ú 2 Now we graph x - y Ú 2 in the same rectangular coordinate system. First we graph the line x - y = 2 using its x-intercept, 2, and its y-intercept, -2. Because (0, 0) makes the inequality x - y Ú 2 false (we obtain 0 Ú 2), we shade the half-plane below the line. This is shown in Figure 8.22 using green vertical shading. The solution of the system is shown in Figure 8.23 by the intersection (the overlap) of the solid yellow and green vertical shadings. The graph of the system’s solution set consists of the region enclosed by the parabola and the line. To find the points of intersection of the parabola and the line, use the substitution method to solve the nonlinear system b y = x2 - 4 x - y = 2. Take a moment to show that the solutions are 1-1, -32 and (2, 0), as shown in Figure 8.23. Check Point 7 Graph the solution set of the system: b y Ú x2 - 4 x + y … 2. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 802 802 Chapter 8 Systems of Equations and Inequalities A system of inequalities has no solution if there are no points in the rectangular coordinate system that simultaneously satisfy each inequality in the system. For example, the system 2x + 3y Ú 6 b 2x + 3y … 0, y 5 4 3 2 1 −5 −4 −3 −2 −1−1 −2 −3 −4 −5 2x + 3y ≥ 6 x 1 2 3 4 5 2x + 3y ≤ 0 whose separate graphs are shown in Figure 8.24, has no overlapping region. Thus, the system has no solution. The solution set is , the empty set. Graphing a System of Inequalities EXAMPLE 8 Graph the solution set of the system: Figure 8.24 A system of x - y 6 2 c -2 … x 6 4 y 6 3. inequalities with no solution Solution We begin by graphing x - y 6 2, the first given inequality. The line x - y = 2 has an x-intercept of 2 and a y-intercept of -2. The test point (0, 0) makes the inequality x - y 6 2 true, and its graph is shown in Figure 8.25. Now, let’s consider the second given inequality, - 2 … x 6 4. Replacing the inequality symbols by =, we obtain x = - 2 and x = 4, graphed as red vertical lines in Figure 8.26. The line of x = 4 is not included. Because x is between - 2 and 4, we shade the region between the vertical lines. We must intersect this region with the yellow region in Figure 8.25. The resulting region is shown in yellow and green vertical shading in Figure 8.26. Finally, let’s consider the third given inequality, y 6 3. Replacing the inequality symbol by =, we obtain y = 3, which graphs as a horizontal line. Because of the less than symbol in y 6 3, the graph consists of the half-plane below the line y = 3. We must intersect this half-plane with the region in Figure 8.26. The resulting region is shown in yellow and green vertical shading in Figure 8.27. This region represents the graph of the solution set of the given system. y y 5 4 3 2 1 −5 −4 −3 −2 −1−1 −2 −3 −4 −5 x = −2 1 2 3 4 5 x−y=2 x −5 −4 −3 y x=4 5 4 3 2 1 −1−1 −2 −3 −4 −5 x = −2 y=3 1 2 3 5 x −5 −4 −3 −2 −1−1 −2 −3 −4 −5 x−y=2 x=4 5 4 3 2 1 1 2 3 4 5 x x−y=2 Figure 8.25 The graph of Figure 8.26 The graph of Figure 8.27 The graph of x - y 6 2 x - y 6 2 x - y 6 2 and -2 … x 6 4 and - 2 … x 6 4 and y 6 3 In Figure 8.27 it may be difficult to tell where the graph of x - y = 2 intersects the vertical line x = 4. Using the substitution method, it can be determined that this intersection point is (4, 2). Take a moment to verify that the four intersection points in Figure 8.27 are, clockwise from upper left, 1- 2, 32, (4, 3), (4, 2), and 1- 2, -42. These points are shown as open dots because none satisfies all three of the system’s inequalities. Check Point 8 Graph the solution set of the system: x + y 6 2 c -2 … x 6 1 y 7 - 3. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 803 Section 8.5 Systems of Inequalities 803 Exercise Set 8.5 Practice Exercises 47. b x2 + y2 … 16 x + y 7 2 48. b x2 + y2 … 4 x + y 7 1 49. b x2 + y2 7 1 x2 + y2 6 16 50. b x2 + y2 7 1 x2 + y2 6 9 51. b 1x - 122 + 1y + 122 6 25 1x - 122 + 1y + 122 Ú 16 52. b 1x + 122 + 1y - 122 6 16 1x + 122 + 1y - 122 Ú 4 53. b x2 + y2 … 1 y - x2 7 0 54. b x2 + y2 6 4 y - x2 Ú 0 55. b x2 + y2 6 16 y Ú 2x 56. b x2 + y2 … 16 y 6 2x In Exercises 1–26, graph each inequality. 1. x + 2y … 8 2. 3x - 6y … 12 3. x - 2y 7 10 5. y … 4. 2x - y 7 4 1 x 3 6. y … 7. y 7 2x - 1 1 x 4 8. y 7 3x + 2 9. x … 1 10. x … - 3 11. y 7 1 12. y 7 - 3 13. x2 + y2 … 1 14. x2 + y2 … 4 15. x2 + y2 7 25 16. x2 + y2 7 36 17. 1x - 22 + 1y + 12 6 9 2 2 18. 1x + 222 + 1y - 122 6 16 19. y 6 x2 - 1 20. y 6 x2 - 9 21. y Ú x2 - 9 22. y Ú x2 - 1 23. y 7 2 x 24. y … 3x 25. y Ú log21x + 12 26. y Ú log31x - 12 In Exercises 27–62, graph the solution set of each system of inequalities or indicate that the system has no solution. 27. b 3x + 6y … 6 2x + y … 8 28. b x - y Ú 4 x + y … 6 29. b 2x - 5y … 10 3x - 2y 7 6 30. b 2x - y … 4 3x + 2y 7 - 6 y 7 2x - 3 31. b y 6 -x + 6 y 6 - 2x + 4 32. b y 6 x - 4 33. b x + 2y … 4 y Ú x - 3 34. b x + y … 4 y Ú 2x - 4 35. b x … 2 y Ú -1 36. b x … 3 y … -1 37. - 2 … x 6 5 38. - 2 6 y … 5 39. b x - y … 1 x Ú 2 40. b 4x - 5y Ú - 20 x Ú -3 41. b x + y 7 4 x + y 6 -1 42. b x + y 7 3 x + y 6 -2 43. b x + y 7 4 x + y 7 -1 44. b x + y 7 3 x + y 7 -2 45. b y Ú x2 - 1 x - y Ú -1 46. b y Ú x2 - 4 x - y Ú 2 x - y … 2 57. c x 7 - 2 y … 3 3x + y … 6 58. c x 7 - 2 y … 4 x Ú 0 y Ú 0 59. d 2x + 5y 6 10 3x + 4y … 12 x Ú 0 y Ú 0 60. d 2x + y 6 4 2x - 3y … 6 3x + y … 6 2x - y … -1 61. d x 7 -2 y 6 4 2x + y … 6 x + y 7 2 62. d 1 … x … 2 y 6 3 Practice Plus In Exercises 63–64, write each sentence as an inequality in two variables. Then graph the inequality. 63. The y-variable is at least 4 more than the product of -2 and the x-variable. 64. The y-variable is at least 2 more than the product of - 3 and the x-variable. In Exercises 65–68, write the given sentences as a system of inequalities in two variables. Then graph the system. 65. The sum of the x-variable and the y-variable is at most 4. The y-variable added to the product of 3 and the x-variable does not exceed 6. 66. The sum of the x-variable and the y-variable is at most 3. The y-variable added to the product of 4 and the x-variable does not exceed 6. 67. The sum of the x-variable and the y-variable is no more than 2. The y-variable is no less than the difference between the square of the x-variable and 4. 68. The sum of the squares of the x-variable and the y-variable is no more than 25. The sum of twice the y-variable and the x-variable is no less than 5. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 804 804 Chapter 8 Systems of Equations and Inequalities In Exercises 69–70, rewrite each inequality in the system without absolute value bars. Then graph the rewritten system in rectangular coordinates. ƒxƒ … 2 69. b ƒyƒ … 3 ƒxƒ … 1 70. b ƒyƒ … 2 The graphs of solution sets of systems of inequalities involve finding the intersection of the solution sets of two or more inequalities. By contrast, in Exercises 71–72, you will be graphing the union of the solution sets of two inequalities. 71. Graph the union of y 7 32 x - 2 and y 6 4. 77. Show that point A is a solution of the system of inequalities that describes healthy weight for this age group. 78. Show that point B is a solution of the system of inequalities that describes healthy weight for this age group. 79. Is a person in this age group who is 6 feet tall weighing 205 pounds within the healthy weight region? 80. Is a person in this age group who is 5 feet 8 inches tall weighing 135 pounds within the healthy weight region? 81. Many elevators have a capacity of 2000 pounds. 72. Graph the union of x - y Ú - 1 and 5x - 2y … 10. a. If a child averages 50 pounds and an adult 150 pounds, write an inequality that describes when x children and y adults will cause the elevator to be overloaded. Without graphing, in Exercises 73–76, determine if each system has no solution or infinitely many solutions. b. Graph the inequality. Because x and y must be positive, limit the graph to quadrant I only. 73. b 3x + y 6 9 3x + y 7 9 74. b 6x - y … 24 6x - y 7 24 75. b 1x + 422 + 1y - 322 … 9 1x + 422 + 1y - 322 Ú 9 76. b c. Select an ordered pair satisfying the inequality. What are its coordinates and what do they represent in this situation? 82. A patient is not allowed to have more than 330 milligrams of cholesterol per day from a diet of eggs and meat. Each egg provides 165 milligrams of cholesterol. Each ounce of meat provides 110 milligrams. a. Write an inequality that describes the patient’s dietary restrictions for x eggs and y ounces of meat. 1x - 422 + 1y + 322 … 24 1x - 422 + 1y + 322 Ú 24 b. Graph the inequality. Because x and y must be positive, limit the graph to quadrant I only. c. Select an ordered pair satisfying the inequality.What are its coordinates and what do they represent in this situation? Application Exercises The figure shows the healthy weight region for various heights for people ages 35 and older. Healthy Weight Region for Men and Women, Ages 35 and Older y a. Write a system of inequalities that models the following conditions: 240 220 B 5.3x − y = 180 200 Weight (pounds) 83. On your next vacation, you will divide lodging between large resorts and small inns. Let x represent the number of nights spent in large resorts. Let y represent the number of nights spent in small inns. 180 You want to stay at least 5 nights. At least one night should be spent at a large resort. Large resorts average $200 per night and small inns average $100 per night. Your budget permits no more than $700 for lodging. Healthy Weight Region 160 A 140 120 b. Graph the solution set of the system of inequalities in part (a). 4.1x − y = 140 100 x 60 62 64 66 68 70 72 Height (inches) 74 76 78 Source: U.S. Department of Health and Human Services If x represents height, in inches, and y represents weight, in pounds, the healthy weight region can be modeled by the following system of linear inequalities: b 5.3x - y Ú 180 4.1x - y … 140. Use this information to solve Exercises 77–80. c. Based on your graph in part (b), what is the greatest number of nights you could spend at a large resort and still stay within your budget? 84. A person with no more than $15,000 to invest plans to place the money in two investments. One investment is high risk, high yield; the other is low risk, low yield. At least $2000 is to be placed in the high-risk investment. Furthermore, the amount invested at low risk should be at least three times the amount invested at high risk. Find and graph a system of inequalities that describes all possibilities for placing the money in the high- and low-risk investments. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 805 Section 8.5 Systems of Inequalities 805 The graph of an inequality in two variables is a region in the rectangular coordinate system. Regions in coordinate systems have numerous applications. For example, the regions in the following two graphs indicate whether a person is obese, overweight, borderline overweight, normal weight, or underweight. Females Males 35 35 Obese 30 Overweight Borderline Overweight 25 Normal Range 20 Body-Mass Index (BMI) Body-Mass Index (BMI) Obese 30 Overweight Borderline Overweight 25 Normal Range 20 Underweight Underweight 15 10 20 30 40 50 Age 60 70 80 15 10 20 30 40 50 Age 60 70 80 Source: Centers for Disease Control and Prevention The horizontal axis shows a person’s age. The vertical axis shows that person’s body-mass index (BMI), computed using the following formula: 703W . BMI = H2 The variable W represents weight, in pounds. The variable H represents height, in inches. Use this information to solve Exercises 85–86. 85. A man is 20 years old, 72 inches (6 feet) tall, and weighs 200 pounds. a. Compute the man’s BMI. Round to the nearest tenth. b. Use the man’s age and his BMI to locate this information as a point in the coordinate system for males. Is this person obese, overweight, borderline overweight, normal weight, or underweight? 86. A woman is 25 years old, 66 inches (5 feet, 6 inches) tall, and weighs 105 pounds. a. Compute the woman’s BMI. Round to the nearest tenth. b. Use the woman’s age and her BMI to locate this information as a point in the coordinate system for females. Is this person obese, overweight, borderline overweight, normal weight, or underweight? 95. Explain how to graph the solution set of a system of inequalities. 96. What does it mean if a system of linear inequalities has no solution? Technology Exercises Graphing utilities can be used to shade regions in the rectangular coordinate system, thereby graphing an inequality in two variables. Read the section of the user’s manual for your graphing utility that describes how to shade a region. Then use your graphing utility to graph the inequalities in Exercises 97–102. 97. y … 4x + 4 98. y Ú 2 x - 2 3 99. y Ú x2 - 4 100. y Ú 1 2 x - 2 2 101. 2x + y … 6 102. 3x - 2y Ú 6 103. Does your graphing utility have any limitations in terms of graphing inequalities? If so, what are they? 104. Use a graphing utility with a 冷 SHADE 冷 feature to verify any five of the graphs that you drew by hand in Exercises 1–26. Writing in Mathematics 105. Use a graphing utility with a 冷 SHADE 冷 feature to verify any five of the graphs that you drew by hand for the systems in Exercises 27–62. 87. What is a linear inequality in two variables? Provide an example with your description. Critical Thinking Exercises 88. How do you determine if an ordered pair is a solution of an inequality in two variables, x and y? 89. What is a half-plane? 90. What does a solid line mean in the graph of an inequality? 91. What does a dashed line mean in the graph of an inequality? 92. Compare the graphs of 3x - 2y 7 6 and 3x - 2y … 6. Discuss similarities and differences between the graphs. 93. What is a system of linear inequalities? 94. What is a solution of a system of linear inequalities? Make Sense? In Exercises 106–109, determine whether each statement makes sense or does not make sense, and explain your reasoning. 106. When graphing a linear inequality, I should always use (0, 0) as a test point because it’s easy to perform the calculations when 0 is substituted for each variable. 107. When graphing 3x - 4y 6 12, it’s not necessary for me to graph the linear equation 3x - 4y = 12 because the inequality contains a 6 symbol, in which equality is not included. A-BLTZMC08_747-820-hr 25-09-2008 10:01 Page 806 806 Chapter 8 Systems of Equations and Inequalities 108. The reason that systems of linear inequalities are appropriate for modeling healthy weight is because guidelines give healthy weight ranges, rather than specific weights, for various heights. 109. I graphed the solution set of y Ú x + 2 and x Ú 1 without using test points. Preview Exercises Exercises 116–118 will help you prepare for the material covered in the next section. 116. a. Graph the solution set of the system: In Exercises 110–113, write a system of inequalities for each graph. 110. 111. y y d 2 2 2 −2 x 4 −4 −2 −2 2 4 b. List the points that form the corners of the graphed region in part (a). x c. Evaluate 3x + 2y at each of the points obtained in part (b). −4 −4 x … 8 y Ú 5. 4 4 −5 −3 x + y Ú 6 117. a. Graph the solution set of the system: 112. 113. y 5 4 3 2 1 y = x2 −4 −3 −2 −1−1 1 2 3 4 x −2 −3 x y d 3x - 2x y y 8 7 6 5 4 3 2 1 x 114. Write a system of inequalities whose solution set includes every point in the rectangular coordinate system. 115. Sketch the graph of the solution set for the following system of inequalities: y Ú nx + b 1n 6 0, b 7 02 y … mx + b 1m 7 0, b 7 02. Section 8.6 118. Bottled water and medical supplies are to be shipped to survivors of an earthquake by plane.The bottled water weighs 20 pounds per container and medical kits weigh 10 pounds per kit. Each plane can carry no more than 80,000 pounds. If x represents the number of bottles of water to be shipped per plane and y represents the number of medical kits per plane, write an inequality that models each plane’s 80,000-pound weight restriction. West Berlin children at Tempelhof airport watch fleets of U.S. airplanes bringing in supplies to circumvent the Soviet blockade. The airlift began June 28, 1948 and continued for 15 months. � Write an objective function � c. Evaluate 2x + 5y at each of the points obtained in part (b). Linear Programming Objectives � 0 0 6 - x + 7. b. List the points that form the corners of the graphed region in part (a). 1 2 3 4 5 6 7 8 b Ú Ú … … describing a quantity that must be maximized or minimized. Use inequalities to describe limitations in a situation. Use linear programming to solve problems. T he Berlin Airlift (1948 – 1949) was an operation by the United States and Great Britain in response to military action by the former Soviet Union: Soviet troops closed all roads and rail lines between West Germany and Berlin, cutting off supply routes to the city. The Allies used a mathematical technique developed during World

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