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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Linear Programming: An Algebraic Approach 4 The Simplex Method with Standard Maximization Problems Standard Minimization Problems Nonstandard Problems

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc The Simplex Method The simplex method is an iterative process. Starting at some initial feasible solution (a corner point – usually the origin), each iteration moves to another corner point with an improved (or at least not worse) value of the objective function. Iteration stops when an optimal solution (if it exists) is found.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc A Standard (maximization) Linear Programming Problem: 1.The objective function is to be maximized. 2.All the variables involved in the problem are nonnegative. 3.Each constraint may be written so that the expression with the variables is less than or equal to a nonnegative constant.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Maximize P = 4x + 5y Subject to Ex. A standard maximization problem: First introduce nonnegative slack variables to make equations out of the inequalities: Next, rewrite the objective function Set = 0 with a 1 on P: –4x – 5y + P = 0

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Form the system: Write as a tableau: Nonbasic variables Basic variables Variables in non-unit columns are given a value of zero, so initially x = y = 0.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Choose a pivot: 1.Select column: select most negative entry in the last row (to left of vertical line). 2.Select row: select smallest ratio: constant/entry (using only entries from selected column) Ratios: Next using the pivot, create a unit column

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc

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Repeat steps Ratios:

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc All entries in the last row are nonnegative therefore an optimal solution has been reached: Assign 0 to variables w/out unit columns (u, v). Notice x = 1, y = 5, and P = 25 (the max).

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc The Simplex Method: 1.Set up the initial simplex tableau. 2.If all entries in the last row are nonnegative then an optimal solution has been reached, go to step 4. 3.Perform the pivot operation: convert pivot to a 1, then use row operations to make a unit column. Return to step 2. 4.Determine the optimal solution(s). The value of the variable heading each unit column is given by the corresponding value in the column of constants. Variables heading the non-unit columns have value zero.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Multiple Solutions There are infinitely many solutions if and only if the last row to the right of the vertical line of the final simplex tableau has a zero in a non-unit column. No Solution A linear programming problem will have no solution if the simplex method breaks down (ex. if at some stage there are no nonnegative ratios for computation).

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Minimization with Constraints Ex. Minimize C = –4 x – 5y Subject to Notice if we let P = –C = 4x + 5y we have a standard maximization problem. If we solve this associated problem we find P = 25, therefore the minimum is C = –25.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc A Standard (minimization) Linear Programming Problem: 1.The objective function is to be minimized. 2.All the variables involved in the problem are nonnegative. 3.Each constraint may be written so that the expression with the variables is greater than or equal to a nonnegative constant.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Maximization problems can be associated with minimization problems (and vice versa). The original problem is called the Primal and the associated problem is called the Dual. The Dual Problem Theorem of Duality A primal problem has a solution if and only if the dual has a solution. Both objective functions attain the same optimal value. The optimal solution of the primal appears under the slack variables in the last row of the final simplex tableau associated with the dual.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Ex.Minimize C = 10x + 11y Subject to Write down the primal information:

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Interchange the columns and rows and use variables u, v, w. This can be represented by the problem: Maximize P = 300u + 300v + 250w Subject to This is a standard maximization problem

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Create the initial simplex tableau adding slack variables x and y. pivot

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc

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This is the final tableau with x = 15, y = 5, and P = C = 205.

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Maximize P = 8x + 3y Subject to Ex. A nonstandard maximization problem: First change the inequalities to less than or equal to. Now proceed with the simplex method

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Introduce slack variables to make equations out of the inequalities and set the objective function = 0: The initial tableau and notice v = –2 (not feasible): We need to pivot to a feasible solution

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Locate any negative number in the constant column ( –2). Now go to the first negative to the left of that constant (–1). This determines the pivot column. The pivot row is found by examining the positive ratios. So –1 is our pivot. Ratios 8 2 Create unit column

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc Note: now we have a feasible solution proceed with simplex New pivot since it is the only positive ratio

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc This is the final tableau: x = 2, y = 4, P = 28

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Copyright (c) 2003 Brooks/Cole, a division of Thomson Learning, Inc The Simplex Method for Nonstandard Problems 1.If necessary, rewrite as a maximization problem. 2.If necessary, rewrite inequalities as less or equal to. 3.Introduce slack variables and write simplex tableau. 4.If no negative constants (upper column) use simplex method, otherwise go to step 5. 5.Pick a negative entry in a row with a negative constant (this is the pivot column). Compute positive ratios to determine pivot row. Then pivot and return to step 4.

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