- 1. Solving Inequalities Algebra I By Ita Rodríguez
- 2. Solving Inequalities • An inequality is a mathematical sentence that uses inequality symbols (<, >, ≤ , ≥) to compare two expressions. • When you use an expression such as at least or at most, you are talking about an inequality. You can use inequalities to represent situations that involves minimum or maximum amounts. • Equations have a definite solution. Inequalities have infinite number of solutions. • Since it is impossible to list an infinite number of solutions, a number line graph is used as means of picturing them.
- 3. Inequalities and Their Graphs Example: • 3 < 5 3 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 5 • 0 > −2 0 𝑖𝑠 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 − 2 • 5 ≤ 6 5 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 6 • 5 ≤ 5 5 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 5 • 7 ≥ −1 7 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 − 1 • 7 ≥ 7 7 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 7
- 4. Inequalities and Their Graphs The inequalities ≤ 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 𝑎𝑛𝑑 ≥ 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 have two parts to them. The inequality will be true if it satisfies one of its parts, not both. Example: • 5 ≤ 6 𝑇ℎ𝑖𝑠 𝑖𝑛𝑒𝑞𝑢𝑎𝑙𝑖𝑡𝑦 𝑖𝑠 𝑡𝑟𝑢𝑒 𝑏𝑒𝑐𝑎𝑢𝑠𝑒 5 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 6. • 5 ≤ 5 𝑇ℎ𝑖𝑠 𝑖𝑛𝑒𝑞𝑢𝑎𝑙𝑖𝑡𝑦 𝑖𝑠 𝑡𝑟𝑢𝑒 𝑏𝑒𝑐𝑎𝑢𝑠𝑒 5 𝑖𝑠 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 5. • 7 ≥ −1 𝑇ℎ𝑖𝑠 𝑖𝑛𝑒𝑞𝑢𝑎𝑙𝑖𝑡𝑦 𝑖𝑠 𝑡𝑟𝑢𝑒 𝑏𝑒𝑐𝑎𝑢𝑠𝑒 7 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 − 1. • 7 ≥ 7 𝑇ℎ𝑖𝑠 𝑖𝑛𝑒𝑞𝑢𝑎𝑙𝑖𝑡𝑦 𝑖𝑠 𝑡𝑟𝑢𝑒 𝑏𝑒𝑐𝑎𝑢𝑠𝑒 7 𝑖𝑠 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 7.
- 5. Inequalities and Their Graphs • What inequality represents the verbal expression? All real numbers 𝑥 less than or equal to −7. Example 1: 𝑥 ≤ −7 All real numbers 𝑥 less than or equal to −7
- 6. Inequalities and Their Graphs • What inequality represents the verbal expression? 6 less than a number 𝑘 is greater than 13. Example 2: 𝑘 − 6 > 13 6 less than a number 𝑘 is greater than 13
- 7. Inequalities and Their Graphs • Is the following number a solution of 2𝑥 + 1 > −3? a) −3 Begin by substituting the value in the inequality. 2 −3 + 1 > −3 Use order of operations to simplify. −6 + 1 > −3 Verify if the inequality is true. −5 > −3 Not true Since −5 is not greater than −3, −3 is not a solution of the inequality. Example 3: A solution of an inequality is any number that makes the inequality true. For that reason, there are many solutions to an inequality, not just one.
- 8. Inequalities and Their Graphs • Is the following number a solution of 2𝑥 + 1 > −3? b) 1 Begin by substituting the value in the inequality. 2 1 + 1 > −3 Use order of operations to simplify. 2 + 1 > −3 Verify if the inequality is true. 3 > −3 True Since 3 is greater than −3, 1 is a solution of the inequality. Example 4:
- 9. Inequalities and Their Graphs A graph can indicate all of the solutions of an inequality. 𝑛 < 1 (𝑛 𝑖𝑠 𝑙𝑒𝑠𝑠 𝑡ℎ𝑎𝑛 1) −3 − 2 − 1 0 1 2 3 First draw a number line. Next, ask yourself if 1 can be part of the solution. No, so draw an open circle of the number 1 to indicate it isn’t. Then ask yourself in which direction are the numbers that are less than 1. Smaller numbers are to the left, so draw a line from the circle to the left.
- 10. Inequalities and Their Graphs 𝑎 ≥ 𝑜 (𝑎 𝑖𝑠 𝑔𝑟𝑒𝑎𝑡𝑒𝑟 𝑡ℎ𝑎𝑛 𝑜𝑟 𝑒𝑞𝑢𝑎𝑙 𝑡𝑜 0) −3 − 2 − 1 0 1 2 3 First draw a number line. Next, ask yourself if 0 can be part of the solution. Yes, so draw a closed circle of the number 0 to indicate it is. Then ask yourself in which direction are the numbers that are greater than 0. Bigger numbers are to the right, so draw a line from the circle to the right.
- 11. Inequalities and Their Graphs Inequality Graph Explanation 𝑛 < 1 −3 − 2 − 1 0 1 2 3 The open circle indicates that 1 is not part of the solution. The line indicates that all the numbers to the left are. 𝑎 ≥ 𝑜 −3 − 2 − 1 0 1 2 3 The closed circle indicates that 0 is part of the solution along with all the numbers to the right. 𝑓 > −3 −3 − 2 − 1 0 1 2 3 The open circle indicates that −3 is not part of the solution. The line indicates that all the numbers to the right are. −2 ≥ 𝑥 −3 − 2 − 1 0 1 2 3 The closed circle indicates that −2 is part of the solution along with all the numbers to the left.
- 12. Inequalities and Their Graphs What inequality represents the graph? −3 − 2 − 1 0 1 2 3 First ask yourself if the circled number is included with a closed circle. Yes, the −2 is included so the inequality will either be ≤ 𝑜𝑟 ≥. Then ask yourself if the numbers that are included with the line are less than −2 or greater than −2 . The shaded numbers are greater than −2 . Now choose any letter for your variable. The answer is 𝑓 ≥ −2. Example 5:
- 13. Inequalities and Their Graphs What inequality represents the graph? −3 − 2 − 1 0 1 2 3 First ask yourself if the circled number is included with a closed circle. No, the 3 is not included so the inequality will either be < 𝑜𝑟 >. Then ask yourself if the numbers that are included with the line are less than 3 or greater than 3. The shaded numbers are less than 3. Now choose any letter for your variable. The answer is 𝑚 < 3. Example 6:
- 14. Solving One-Step Inequalities by Adding or Subtracting Solve 𝑛 − 10 > 14. Graph the solution. Begin by asking yourself what is being done to the variable. Subtracting 10 What is the inverse (opposite) of subtracting 10? Adding 10 Now add 10 to both sides of the inequality. Example 7: One-step inequalities are solved the same way one-step equations are solved (by using the inverse operation).
- 15. Solving One-Step Inequalities by Adding or Subtracting Solve 𝑛 − 10 > 14. Graph the solution. 𝑛 − 10 > 14 +10 + 10 24 𝑛 > 24 Example 7:
- 16. Solving One-Step Inequalities by Adding or Subtracting Solve 𝑛 − 10 > 14. Graph the solution. 𝑛 > 24 First draw the number line placing the 24 in the center. Then, follow the steps for graphing. 23 24 25 Example 7:
- 17. Solving One-Step Inequalities by Adding or Subtracting A club has to sell at least 25 plants for a fund raiser. Club members sell 8 plants on Wednesday and 9 plants on Thursday. What are the possible numbers of plants the club can sell on Friday to meet their goal? First look for key words to relate the information given. At least means that 25 is the smallest possible number. Therefore, it means that the numbers that are allowed are 25, 26, 27, 28, 29, … So at least is represented by the greater than or equal to symbol ≥. Example 8: ≥ 258 + 9 𝑓+
- 18. Solving One-Step Inequalities by Adding or Subtracting A club has to sell at least 25 plants for a fund raiser. Club members sell 8 plants on Wednesday and 9 plants on Thursday. What are the possible numbers of plants the club can sell on Friday to meet their goal? Now solve the inequality the same way you solved in example 7. 8 + 9 + 𝑓 ≥ 25 17 + 𝑓 ≥ 25 −17 − 17 𝑓 ≥ 8 Example 8:
- 19. Solving One-Step Inequalities by Adding or Subtracting A club has to sell at least 25 plants for a fund raiser. Club members sell 8 plants on Wednesday and 9 plants on Thursday. What are the possible numbers of plants the club can sell on Friday to meet their goal? 𝑓 ≥ 8 The possible number of plants the club can sell on Friday to meet their goals are greater than or equal to 8. Example 8:
- 20. Solving One-Step Inequalities by Multiplying or Dividing What are the solutions to 𝑥 3 ≤ −2? Graph the solution. Begin by asking yourself what is being done to the variable. Dividing 3 What is the inverse (opposite) of dividing 3? Multiplying 3 Now multiply 3 to both sides of the inequality. Example 9:
- 21. Solving One-Step Inequalities by Multiplying or Dividing What are the solutions to 𝑥 3 ≤ −2? Graph the solution. 𝑥 3 ≤ −2 3 ∙ 𝑥 3 ≤ −2 ∙ 3 𝑥 ≤ −6 Example 9:
- 22. Solving One-Step Inequalities by Multiplying or Dividing What are the solutions to 𝑥 3 ≤ −2? Graph the solution. 𝑥 ≤ −6 −7 − 6 − 5 Example 9:
- 23. Solving One-Step Inequalities by Multiplying or Dividing You walk dogs in your neighborhood after school. You earn $4.50 per dog. How many dogs do you need to walk to earn at least $75? Round to whole numbers. First look for key words to relate the information given. $4.50 per dog means multiply $4.50 to every dog you walk. At least means that $75 is the smallest possible number. Therefore, it means that the numbers that are allowed are 75, 76, 77, 78, 79, … So at least is represented by the greater than or equal to symbol ≥. Example 10: ≥ 75𝑑4.50
- 24. Solving One-Step Inequalities by Multiplying or Dividing You walk dogs in your neighborhood after school. You earn $4.50 per dog. How many dogs do you need to walk to earn at least $75? Round to whole numbers. Now solve the inequality. 4.50𝑑 ≥ 75 4.50𝑑 4.50 ≥ 75 4.50 𝑑 ≥ 16.67 𝑑 ≥ 17 Example 10:
- 25. Solving One-Step Inequalities by Multiplying or Dividing You walk dogs in your neighborhood after school. You earn $4.50 per dog. How many dogs do you need to walk to earn at least $75? Round to whole numbers. 𝑑 ≥ 17 You need to walk at least 17 dogs to earn at least $75. You cannot walk at least 16.67 dog because you cannot have part of a dog. Example 10:
- 26. Solving One-Step Inequalities by Multiplying or Dividing You have already seen solving inequalities using multiplication or division of a positive number. Solving inequalities using multiplication or division of a negative number is different. 3 < 4 When you multiply 2 to both sides, the inequality remains true. 2 ∙ 3 < 2 ∙ 4 6 < 8 True However, if you multiply −2 to both sides, the inequality becomes false. −2 ∙ 3 < −2 ∙ 4 −6 < −8 False
- 27. Solving One-Step Inequalities by Multiplying or Dividing Therefor, the rule of solving inequalities when multiplying or dividing by a negative number is to reverse (or flip) the direction of the inequality symbol. 3 < 4 −2 ∙ 3 < −2 ∙ 4 −6 > −8 True
- 28. Solving One-Step Inequalities by Multiplying or Dividing What are the solutions to − 3 4 𝑤 > 3? Graph the solution. Begin by asking yourself what is being done to the variable. Multiplying − 3 4 What is the inverse (opposite) of multiplying − 3 4 ? Dividing − 3 4 which really means to multiply the reciprocal − 4 3 Now multiply − 4 3 to both sides of the inequality. Example 11:
- 29. Solving One-Step Inequalities by Multiplying or Dividing What are the solutions to − 3 4 𝑤 > 3? Graph the solution. − 3 4 𝑤 > 3 − 4 3 ∙ − 3 4 𝑤 > 3 ∙ − 4 3 Flip the symbol 𝑤 < − 12 3 Example 11:
- 30. Solving One-Step Inequalities by Multiplying or Dividing What are the solutions to − 3 4 𝑤 > 3? Graph the solution. 𝑥 < − 12 3 𝑤 < −4 −5 − 4 − 3 Example 11:
- 31. Solving Multi-Step Inequalities You solve multi-step inequalities the same way you solve multi-step equations. Use the inverse of the order of operations. Remember to reverse (flip) the symbol if you multiply or divide by a negative number.
- 32. Solving Multi-Step Inequalities What are the solutions to −4 ≤ 5 − 3𝑛? −4 ≤ 5 − 3𝑛 Begin by looking for an addition or subtraction. 5 is being added so the inverse is to subtract 5. −4 ≤ 5 − 3𝑛 −5 − 5 −9 −9 ≤ −3𝑛 Example 12:
- 33. Solving Multi-Step Inequalities What are the solutions to −4 ≤ 5 − 3𝑛? −9 ≤ −3𝑛 Next, look for a multiplication or division. −3 is being multiplied so the inverse is to divide −3. Since the number you are dividing is negative, you will have to flip the symbol. −9 −3 ≤ −3𝑛 −3 3 ≥ 𝑛 Example 12:
- 34. Solving Multi-Step Inequalities What are the solutions to −4 ≤ 5 − 3𝑛? 3 ≥ 𝑛 You can rewrite the inequality with the variable first. Notice how the symbol points to the variable, so when you rewrite it, make sure the symbol still points to the variable. 𝑛 ≤ 3 Example 12:
- 35. Solving Multi-Step Inequalities What are the solutions to 10 − 8𝑎 ≥ 2(5 − 4𝑎)? Begin by distributing the 2. 10 − 8𝑎 ≥ 2(5 − 4𝑎) Next, move the variable to the left side. 10 − 8𝑎 ≥ 10 − 8𝑎 +8𝑎 + 8𝑎 10 ≥ 10 Since the variables cancel out and the inequality remains true, the solution is all real numbers. Example 13:
- 36. Solving Multi-Step Inequalities What are the solutions to 6𝑚 − 5 > 7𝑚 + 7 − 𝑚? Begin by combining like terms. 6𝑚 − 5 > 7𝑚 + 7 − 𝑚 Next, move the variable to the left side. 6𝑚 − 5 > 6𝑚 + 7 −6m − 6𝑚 −5 > 7 Since the variables cancel out and the inequality is false, there is no solution to the inequality. Example 14:
- 37. Solving Multi-Step Inequalities What are the solutions to 5 − 3𝑓 > 2(4𝑓 − 3)? Begin by distributing the 2. 5 − 3𝑓 > 2(4𝑓 − 3) Next, move the variable to the left side. 5 − 3𝑓 > 8𝑓 − 6 −8𝑓 − 8𝑓 5 − 11𝑓 > −6 Example 15:
- 38. Solving Multi-Step Inequalities What are the solutions to 5 − 3𝑓 > 2(4𝑓 − 3)? Then, move the constant to the right. 5 − 11𝑓 > −6 −5 − 5 −11𝑓 > −11 Example 15:
- 39. Solving Multi-Step Inequalities What are the solutions to 5 − 3𝑓 > 2(4𝑓 − 3)? −11𝑓 > −11 −11 is being multiplied so the inverse is to divide −11. Since the number you are dividing is negative, you will have to flip the symbol. −11𝑓 −11 > −11 −11 𝑓 < 1 Example 15:
- 40. Solving Compound Inequalities A compound inequality consists of two distinct inequalities joined by the word 𝑎𝑛𝑑 or the word 𝑜𝑟. The graph of compound inequality with the word 𝑎𝑛𝑑 contains the overlapping region of two inequalities. 𝑥 > 3 1 2 3 4 5 6 7 𝑥 ≤ 7 1 2 3 4 5 6 7 𝑥 > 3 𝑎𝑛𝑑 𝑥 ≤ 7 1 2 3 4 5 6 7
- 41. Solving Compound Inequalities There is another way to write an 𝑎𝑛𝑑 compound inequality. Guide yourself with the graph. 𝑥 > 3 𝑎𝑛𝑑 𝑥 ≤ 7 1 2 3 4 5 6 7 𝑥 > 3 𝑎𝑛𝑑 𝑥 ≤ 7 is the same as 3 < 𝑥 ≤ 7. 3 < 𝑥 7≤
- 42. Solving Compound Inequalities The graph of compound inequality with the word 𝑜𝑟 contains each graph of the two inequalities. 𝑥 ≤ −1 −3 − 2 − 1 0 1 2 3 𝑥 > 2 −3 − 2 − 1 0 1 2 3 𝑥 ≤ −1 or 𝑥 > 2 −3 − 2 − 1 0 1 2 3
- 43. Solving Compound Inequalities What are the solutions to 3𝑡 + 2 < −7 or −4𝑡 + 5 < 1? Graph the solution. Begin by solving each inequality separately. 3𝑡 + 2 < −7 −4𝑡 + 5 < 1 −2 − 2 −9 3𝑡 < −9 3 3 𝑡 < −9 3 Example 16:
- 44. Solving Compound Inequalities What are the solutions to 3𝑡 + 2 < −7 or −4𝑡 + 5 < 1? Graph the solution. 3 3 𝑡 < −9 3 − 4𝑡 + 5 < 1 𝑡 < −3 Now continue with the second inequality. −4𝑡 + 5 < 1 −5 − 5 −4 Example 16:
- 45. Solving Compound Inequalities What are the solutions to 3𝑡 + 2 < −7 or −4𝑡 + 5 < 1? Graph the solution. 𝑡 < −3 − 4𝑡 + 5 < 1 −5 − 5 −4 −4𝑡 < −4 −4 −4 𝑡 < −4 −4 𝑡 > 1 Example 16:
- 46. Solving Compound Inequalities What are the solutions to 3𝑡 + 2 < −7 or −4𝑡 + 5 < 1? Graph the solution. 𝑡 < −3 or 𝑡 > 1 −4 − 3 − 2 − 1 0 1 2 Example 16:
- 47. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. You can solve this compound inequality two ways. Method one, separate it into two inequalities and work them out as in example 16. Method two, work out its two parts at the same time. Example 17:
- 48. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. Method one: Separate it into two inequalities. 8 ≥ −5𝑥 − 2 > 3 Continue to solve normally. +2 + 2 10 10 ≥ −5𝑥 Example 17: 8 ≥ −5𝑥 − 2 −5𝑥 − 2 > 3
- 49. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. 10 ≥ −5𝑥 −5𝑥 − 2 > 3 10 −5 ≥ −5𝑥 −5 −2 ≤ 𝑥 Now continue with the second inequality. Example 17:
- 50. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. −2 ≤ 𝑥 −5𝑥 − 2 > 3 +2 + 2 5 −5𝑥 > 5 −5𝑥 −5 > 5 −5 𝑥 < −1 Example 17:
- 51. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. −2 ≤ 𝑥 𝑥 < −1 Now put the two inequalities back together following the order of the number line and graph. −2 ≤ 𝑥 < −1 −3 − 2 − 1 0 1 2 3 Example 17:
- 52. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. Method two: Work out its two parts at the same time. 8 ≥ −5𝑥 − 2 > 3 Focus your attention in the center where the variable is. You are subtracting a 2 so the inverse is to add 2. Do it to the three parts. Example 17: 10 +2 +2+2 10 ≥ −5𝑥 > 5 5
- 53. Solving Compound Inequalities What are the solutions to 8 ≥ −5𝑥 − 2 > 3? Graph the solution. Method two: Again, focus your attention in the center where the variable is. You are multiplying −5 so the inverse is to divide −5. Do it to the three parts. Remember to flip the symbols because you are dividing by a negative number. Graph as we did before. Example 17: 10 ≥ −5𝑥 > 5 10 −5 ≥ −5𝑥 −5 > 5 −5 −2 ≤ 𝑥 < −1
- 54. The End