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Digital Lesson Graphing Linear Inequalities in Two Variables Expressions of the type x + 2y ≤ 8 and 3x – y > 6 are called linear inequalities in two variables. A solution of a linear inequality in two variables is an ordered pair (x, y) which makes the inequality true. Example: (1, 3) is a solution to x + 2y ≤ 8 since (1) + 2(3) = 7 ≤ 8. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 2 The solution set, or feasible set, of a linear inequality in two variables is the set of all solutions. y Example: The solution set for x + 2y ≤ 8 is the shaded region. 2 2 x The solution set is a half-plane. It consists of the line x + 2y ≤ 8 and all the points below and to its left. The line is called the boundary line of the half-plane. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 3 If the inequality is ≤ or ≥ , the boundary line is solid; its points are solutions. 3x – y = 2 y x 3x – y < 2 Example: The boundary line of the 3x – y > 2 solution set of 3x – y ≥ 2 is solid. If the inequality is < or >, the boundary line is dotted; its points are not solutions. Example: The boundary line of the solution set of x + y < 2 is dotted. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. y x 4 A test point can be selected to determine which side of the half-plane to shade. y Example: For 2x – 3y ≤ 18 graph the boundary line. (0, 0) -2 2 x The solution set is a half-plane. Use (0, 0) as a test point. (0, 0) is a solution. So all points on the (0, 0) side of the boundary line are also solutions. Shade above and to the left of the line. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 5 To graph the solution set for a linear inequality: 1. Graph the boundary line. 2. Select a test point, not on the boundary line, and determine if it is a solution. 3. Shade a half-plane. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 6 Example: Graph the solution set for x – y > 2. 1. Graph the boundary line x – y = 2 as a dotted line. y (0, 0) x 2. Select a test point not on the line, say (0, 0). (2, 0) (0, -2) (0) – 0 = 0 > 2 is false. 3. Since this is a not a solution, shade in the half-plane not containing (0, 0). Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 7 Solution sets for inequalities with only one variable can be graphed in the same way. y Example: Graph the solution set for x < -2. 4 x -4 4 -4 y Example: Graph the solution set for x ≥ 4. 4 x -4 4 -4 Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 8 A solution of a system of linear inequalities is an ordered pair that satisfies all the inequalities. x y 8 Example: Find a solution for the system . 2 x y 7 (5, 4) is a solution of x + y > 8. (5, 4) is also a solution of 2x – y ≤ 7. Since (5, 4) is a solution of both inequalities in the system, it is a solution of the system. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 9 The set of all solutions of a system of linear inequalities is called its solution set. To graph the solution set for a system of linear inequalities in two variables: 1. Shade the half-plane of solutions for each inequality in the system. 2. Shade in the intersection of the half-planes. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 10 Example: x y 8 Graph the solution set for the system 2 x y 7 y Graph the solution set for x + y > 8. Graph the solution set for 2x – y ≤ 7. The intersection of these two half-planes is the wedge-shaped region at the top of the diagram. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 2 2 x 11 Example: Graph the solution set for the system of linear inequalities: 2 x 3 y 12 2 x 3 y 6 y -2x + 3y ≥ 6 Graph the two half-planes. 2 The two half-planes do not intersect; therefore, the solution set is the empty set. x 2 2x – 3y ≥ 12 Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 12 Example: Graph the solution set for the linear system. (1) 2 x 3 y 3 (2) 6 x y 1 (3) x 2 (4) y 1 (2) y (1) 4 x -4 4 Graph each linear inequality. (4) -4 (3) The solution set is the intersection of all the half-planes. Copyright © by Houghton Mifflin Company, Inc. All rights reserved. 13