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Linear Programming: The Graphical Method

Linear Programming: The Graphical Method. 1. Problem description 2. Formulate the problem as a linear program 3. Plot the constraints 4. Identify the feasible region 5. Find the optimal solution A. Isoprofit line solution method B. Corner point solution method

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Linear Programming: The Graphical Method

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  1. Linear Programming:The Graphical Method • 1. Problem description • 2. Formulate the problem as a linear program • 3. Plot the constraints • 4. Identify the feasible region • 5. Find the optimal solution • A. Isoprofit line solution method • B. Corner point solution method • Interpretation of the solution ADM2302 ~ Rim Jaber

  2. RMC, Inc. is a firm that produces chemical based products. In a particular process three raw materials are used to produce two products. The Material requirements per ton are: Product Material 1 Material 2 Material 3 Fuel additive 2/5 0 3/5 Solvent base 1/2 1/5 3/10 For the current production period RMC has available the following quantities of each raw material. Because of spoilage, any materials not used for current production must be discarded. Number of Tons Material Available for Production Material 1 20 Material 2 5 Material 3 21 If the contribution to the profit is $40 for each ton of fuel additive and $30 for each ton of solvent base, How many tons of each product should be produced in order to maximize the total contribution profit? ADM2302 ~ Rim Jaber

  3. 2. RMC Problem Formulation Max Z = $ 40 x1 + 30 x2 Subject to 2/5 x1 + 1/2 x2  20 (Material 1) 1/5 x2  5 (Material 2) 3/5 x1 + 3/10 x2  21 (Material 3) x1  0, x2  0 x1: number of tons of fuel additive that RMC produces x2: number of tons of solvent base that RMC produces ADM2302 ~ Rim Jaber

  4. 4. The Feasible Region x2 50 40 1/5 x2 = 5 30 2/5x1 + 1/2 x2 = 20 20 3/5 x1 + 3/10 x2= 21 10 x1 30 40 0 20 50 10 ADM2302 ~ Rim Jaber

  5. 5. The Optimal Solution: A.The Isoprofit or Isocost Line Solution Method • Select value for profit or cost, and draw isoprofit / isocost line to reveal its slope • Z = $240 • With a maximization problem, maintain same slope and move line up and right until it touches feasible region at one point. With minimization, move down and left until it touches only one point in feasible region. • Identify optimal solution as coordinates of point touched by highest possible isoprofit line or lowest possible isocost line. • Read optimal coordinates and compute optimal profit or cost. ADM2302 ~ Rim Jaber

  6. A. The Optimal Solution x2 50 Maximum isoprofit line 40 30 Optimal Solution Point 20 10 x1 30 40 0 20 50 10 ADM2302 ~ Rim Jaber

  7. Interpretation of the Solution • The solution indicates that if RMC, Inc. produces 25 tons of fuel additive and 20 tons of solvent base, it will receive $1600, the maximum profit possible given the Material constraints (resources constraints) • The solution tells management that the optimal solution will require all available material 1 and material 3, but only 4 of 5 tons of material 2. (the 1 ton of material 2 is referred as slack ADM2302 ~ Rim Jaber

  8. Corner Point Property Very important property of Linear Programming problems: Corner Point Property. Property states optimal solution to LP problem will alwaysoccur at a corner point. • The solution point will be on the boundary of the feasible solution area and at one of the corners of the boundary where two constraint lines intersect ADM2302 ~ Rim Jaber

  9. 5. The Optimal Solution: B. The Corner Point Solution Methods • Solve simultaneous equations at each corner point to find the solution values at each point. • Substitute these values into the objective function (Z) to find the set of values that results in the maximum Z value ADM2302 ~ Rim Jaber

  10. B. The Optimal Solution x2 50 40 (0, 25) (18.75,25) 30 Optimal Solution Point (25,20) 20 (0,0) (35,0) 10 x1 40 0 20 50 10 30 ADM2302 ~ Rim Jaber

  11. A Minimization LP Problem Many LP problems involve minimizingobjective such as costinstead of maximizing profit function. Examples: • Restaurant may wish to develop work schedule to meet staffing needs whileminimizing total number ofemployees. • Manufacturer may seek to distribute its products from several factories to its many regional warehouses in such a way as tominimize total shipping costs. • Hospital may want to provide its patients with a daily meal plan that meets certain nutritional standards while minimizing food purchase costs. ADM2302 ~ Rim Jaber

  12. Example of a Two Variable Minimization LP Problem Holiday Meal Turkey Ranch • Buy two brands of feed for good, low-cost diet for turkeys. • Each feed may contain three nutritional ingredients (protein, vitamin, and iron). • One pound of Brand A contains: • 5 units of protein, • 4 units of vitamin, and • 0.5 units of iron. • One pound of Brand B contains: • 10 units of protein, • 3 units of vitamins, and • 0 units of iron. ADM2302 ~ Rim Jaber

  13. Example of Two Variable Minimization Linear Programming Problem Holiday Meal Turkey Ranch • Brand A feed costs ranch $0.02 per pound, while Brand B feed costs $0.03 per pound. • Ranch owner would like lowest-cost diet that meetsminimum monthly intake requirementsfor each nutritionalingredient. ADM2302 ~ Rim Jaber

  14. Summary of Holiday Meal Turkey Ranch Data ADM2302 ~ Rim Jaber

  15. Formulation of LP Problem: Minimize cost (in cents) = 2A + 3B Subject to: 5A + 10B 90 (protein constraint) 4A + 3B 48 (vitamin constraint) ½A 1½ (iron constraint) A 0, B 0 (nonnegativity constraint) Where: A denotes number of pounds of Brand A feed, and B denote number of pounds of Brand B feed. ADM2302 ~ Rim Jaber

  16. Graphical Solution of Holiday Meal Turkey Ranch Problem Drawing Constraints: ½A 1½ 4A + 3B 48 Nonnegativity Constraint A 0, B 0 5A + 10B 90 ADM2302 ~ Rim Jaber

  17. Isocost Line Method. One can start by drawing a 54-cent cost line 2A + 3B. = 54 ADM2302 ~ Rim Jaber

  18. Isocost Line Method • Isocost line is moved parallel to 54-cent solution line toward lower left origin. • Last point to touch isocost line while still in contact with feasible region is corner point 2. ADM2302 ~ Rim Jaber

  19. Isocost Line Method • Solving for corner point 2 with two equations with values 8.4 for A and • 4.8 for B, minimum optimal cost solution is: • 2A + 3B = (2)(8.4) + (3)(4.8) = 31.2

  20. Corner Point Solution Method • Point 1 - coordinates (A = 3, B = 12) • cost of 2(3) + 3(12) = 42 cents. • Point 2 - coordinates (A = 8.4, b = 4.8) • cost of 2(8.4) + 3(4.8) = 31.2 cents • Point 3 - coordinates (A = 18, B = 0) • cost of (2)(18) + (3)(0) = 36 cents. • Optimal minimal cost solution: Corner Point 2, cost = 31.2 cents ADM2302 ~ Rim Jaber

  21. Summary of Graphical Solution Methods • Graph each constraint equation. • Identify feasible solution region, that is, area that satisfies all constraints simultaneously. • Select one of two following graphical solution techniques and proceed to solve problem. • Corner Point Method. • Isoprofit or Isocost Method. ADM2302 ~ Rim Jaber

  22. Corner Point Method Determine coordinates of each of corner points of feasible region by solving equations. Compute profit or cost at each point by substitution of values of coordinates into objective function and solving for result. Identify an optimal solution as a corner point with highest profit (maximization problem), or lowest cost (minimization).  Summary of Graphical Solution Methods (Continued) ADM2302 ~ Rim Jaber

  23. Isoprofit or Isocost Method Select value for profit or cost, and draw isoprofit / isocost line to reveal its slope. With a maximization problem, maintain same slope and move line up and right until it touches feasible region at one point. With minimization, move down and left until it touches only one point in feasible region. Identify optimal solution as coordinates of point touched by highest possible isoprofit line or lowest possible isocost line. Read optimal coordinates and compute optimal profit or cost. Summary of Graphical Solution Methods (continued) ADM2302 ~ Rim Jaber

  24. Special Cases in LP • Infeasibility • Unbounded Solutions (Unboundedness) • Redundancy • More Than One Optimal Solution (Alternate Optimal Solutions) ADM2302 ~ Rim Jaber

  25. Infeasibility • A problem with no feasible region because of the presence of conflicting constraints • Consider the following 3 constraints: X1 + 2X2 <= 6 2X1 + X2 <= 8 X1 >= 7 ADM2302 ~ Rim Jaber

  26. A Problem with No Feasible Solution X2 8 6 4 2 0 Region Satisfying 3rd Constraint 2 4 6 8 X1 Region Satisfying First 2 Constraints ADM2302 ~ Rim Jaber

  27. Unboundedness • LP model does not have a finite solution • Consider the following example: Max 3X1 + 5X2 Subject to X1 >= 5 X2 <= 10 X1 + 2X2 >= 10 X1, X2 >= 0 ADM2302 ~ Rim Jaber

  28. A Solution Region That is Unbounded to the Right X2 15 10 5 0 X1 > 5 X2 < 10 Feasible Region X1 +2X2 > 10 5 10 15 X1 ADM2302 ~ Rim Jaber

  29. Redundancy • A redundant constraint is constraint that does not affect feasible region in any way. • Consider the following example: Max X1 + 2X2 Subject to X1 + X2 <= 20 2X1 + X2 <= 30 X1 <= 25 X1, X2 >= 0 ADM2302 ~ Rim Jaber

  30. A Problem with a Redundant Constraint X2 30 25 20 15 10 5 0 Redundant Constraint 2X1 + X2 < 30 X1 < 25 X1 +X2 < 20 Feasible Region X1 5 10 15 20 25 30 ADM2302 ~ Rim Jaber

  31. Alternate Optimal Solutions • An LP problem may have more than oneoptimal solution. • Graphically, when the isoprofit (or isocost)line runs parallel to a constraint in problem which lies in direction in which isoprofit (or isocost) line is located. • In other words, when they have same slope. ADM2302 ~ Rim Jaber

  32. Alternate Optimal Solutions (Cont’d) • Consider the following example: Max 3X1 + 2X2 Subject to 6X1 + 4X2 <= 24 X1 <= 3 X1, X2 >= 0 ADM2302 ~ Rim Jaber

  33. An Example of Alternate Optimal Solutions 8 7 6 5 4 3 2 1 0 Optimal Solution Consists of All Combinations of X1 and X2 Along the AB Segment A Isoprofit Line for $8 Isoprofit Line for $12 Overlays Line Segment B AB 1 2 3 4 5 6 7 8 ADM2302 ~ Rim Jaber

  34. Example: Alternate Optimal Solutions • At profit level of $12, isoprofit line will rest directly on top of first constraint line. • This means that any point along line between corner points 1 and 2 provides an optimal X and Y combination. ADM2302 ~ Rim Jaber

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