Graphing Systems of Inequalities

1. Systems of Inequalities Lesson 6.5

2. Frequently, real-world situations involve a range of possible values. Algebraic statements of these situations are called inequalities.

3. Recall that you can perform operations on inequalities very much like you do on equations. • You can add or subtract the same quantity on both sides, multiply by the same number or expression on both sides, and so on. • The one exception to remember is that when you multiply or divide by a negative quantity or expression, the inequality symbol reverses.

4. In this lesson you will learn how to graphically show solutions to inequalities with two variables, such as the last two statements in the table above.

5. Paying for College • A total of $40,000 has been donated to a college scholarship fund. The administrators of the fund are considering how much to invest in stocks and how much to invest in bonds. Stocks usually pay more but are often a riskier investment, whereas bonds pay less but are usually safer. • Let x represent the amount in dollars invested in stocks, and let y represent the amount in dollars invested in bonds. Graph the equation x + y = 40,000.

6. Name at least five pairs of x- and y-values that satisfy the inequality x + y < 40,000 and plot them on your graph. In this problem, why can x + y be less than $40,000?

7. Describe where all possible solutions to the inequality x + y < 40,000 are located. Shade this region on your graph.

8. Describe some points that fit the condition x +y ≤ 40,000 but do not make sense for the situation.

9. Assume that each option—stocks or bonds—requires a minimum investment of $5,000, and that the fund administrators want to purchase some stocks and some bonds. Based on the advice of their financial advisor, they decide that the amount invested in bonds should be at least twice the amount invested in stocks. • Translate all of the limitations, or constraints, into a system of inequalities. A table might help you to organize this information.

10. Assume that each option—stocks or bonds—requires a minimum investment of $5,000, and that the fund administrators want to purchase some stocks and some bonds. Based on the advice of their financial advisor, they decide that the amount invested in bonds should be at least twice the amount invested in stocks. Assume that each option—stocks or bonds—requires a minimum investment of $5,000, and that the fund administrators want to purchase some stocks and some bonds. Based on the advice of their financial advisor, they decide that the amount invested in bonds should be at least twice the amount invested in stocks.

11. Graph all of the inequalities and determine the region of your graph that will satisfy all the constraints. Find each corner, or vertex, of this region. A (5000, 35000) B (13,333, 26,666.67) C (5000, 10000)

12. When there are one or two variables in an inequality, you can represent the solution as a set of ordered pairs by shading the region of the coordinate plane that contains those points.

13. When you have several inequalities that must be satisfied simultaneously, you have a system. • The solution to a system of inequalities with two variables will be a set of points. This set of points is called a feasible region. • The feasible region can be shown graphically as part of a plane, or sometimes it can be described as a geometric shape with its vertices given.

14. Example A • Rachel has 3 hours to work on her homework tonight. She wants to spend more time working on math than on chemistry, and she must spend at least a half hour working on chemistry. • Let x represent time in hours spent on math, and let y represent time in hours spent on chemistry. Write inequalities to represent the three constraints of the system.

15. Example A • Rachel has 3 hours to work on her homework tonight. She wants to spend more time working on math than on chemistry, and she must spend at least a half hour working on chemistry. • Graph your inequalities and shade the feasible region.

16. Example A • Rachel has 3 hours to work on her homework tonight. She wants to spend more time working on math than on chemistry, and she must spend at least a half hour working on chemistry. • Find the coordinates of the vertices of the feasible region.

17. Example A • Name two points that are solutions to the system, and describe what they mean in the context of the problem. The points (1.5, 1) and (2.5, 0.5) are two solutions to the system. Every point in the feasible region represents a way that Rachel could divide her time. The solution point (1.5, 1) means she could spend 1.5 h on mathematics and 1 h on chemistry and still meet all her constraints. The point (2.5, 0.5) means that Rachel could spend 2.5 h on math and 0.5 h on chemistry. This point represents the boundaries of two constraints: She can’t spend less than 0.5 h on chemistry or more than 3 h total on homework.

18. Example B • Anna throws a ball straight up next to a building. The ball’s height in feet after t seconds is given by -16t2+51t+3. Tom rides a glass elevator down the outside of the same building. His height from the time Anna throws the ball can be expressed as 43-5t. As Tom is riding down, he sees a bird fly by above the elevator but below the ball. When did Tom see the bird? Give a range of possible times.

19. At the time Tom sees the bird, the bird’s height, h, must satisfy 43-5t<h< -16t2+51t+3. • You can graph these two inequalities separately. You might find it easier to graph the boundaries first and then shade the feasible region. Tom saw the bird between 1 and 2.5 seconds after Anna threw the ball.