3. RULES OF CONSTRUCTING DUALITY
For every primal constraint there is a dual variable.
For every primal variable there is a dual constraint.
The constraint coefficients of a primal variable from the left-hand side
coefficients of the corresponding dual constraint and its objective function
coefficient of the same variable becomes the right-hand side of the dual
constraints.
If the primal objective function is maximize (minimize), then the dual problem
is minimize (maximize).
If the primal problem is maximize (minimize), then the dual constraints must
be >= (<=).
All dual variables are originally unrestricted.
4. EXAMPLE 1
Primal
Min Z= 15x1 + 12x2
Subject to: x1 + 2x2 >= 3
2x1 - 4x2 <= 5
x1, x2 >= 0
5. EXAMPLE 1
Standard from
Min Z= 15x1 + 12x2
Subject to: x1 + 2x2 – x3 = 3
2x1 - 4x2 + x4 = 5
x1, x2, x3, x4 >= 0
6. EXAMPLE 1
The dual problem
Max W = 3y1 + 5y2
Subject to: y1 + 2y2 <= 15
2y1 – 4y2 <= 12
-y1 + 0y2 <= 0 y1 >= 0
0y1 + y2 <= 0 y2 <= 0, y2 is unrestricted
8. EXAMPLE 2
Standard form
Max Z= 5x1 + 12x2 +4x3
Subject to: x1 + 2x2 + x3 + x4 = 10
2x1 - x2 + 3x3 + 0x4 = 8
x1, x2, x3, x4 >= 0
9. EXAMPLE 2
The dual problem
Min W= 10y1 + 8y2
Subject to: y1 + 2y2 >= 5
2y1 - y2 >= 12
y1 + 3y2 >= 4
y1 + 0y2 >= 0 y1 >= 0, y2 is unrestricted
10. EXAMPLE 2 BY TWO PHASE METHOD
By two phase method
Min W= 10y1 + 8y2
Subject to: y1 + 2y2 >= 5
2y1 - y2 >= 12
y1 + 3y2 >= 4
y1 + 0y2 >= 0 y1 >= 0, y2 is unrestricted
When y is unrestricted its equal to y’-y’’
11. EXAMPLE 2 BY TWO PHASE METHOD
Chanocial form
Min W= 10y1 + (8y2’ – 8y2’’)
Subject to: y1 + (2y2’ – 2y2’’) – y3 + y6 = 5, (y3 is surplus, y6 is artificial)
2y1 – (y2’ – y2’’) – y4 + y7 = 12, (y4 is surplus, y7 is artificial)
y1 + (3y2’ – 3y2’’) – y5 + y8 = 4, (y5 is surplus, y8 is artificial)
18. OPTIMAL DUAL SOLUTION
How to find the optimal solution of the dual problem, when the optimal
solution of the primal problem is known? By 2 methods.
inverse
primal
Optimal
.
variablesbasicprimaloptimalof
tcoefficienobectiveoriginal
ofvectorRow
variablesdual
ofvaluesOptimal
2Method
xioftcoefficienobjectiveOriginal
xivariablestartingoftcoefficien-zprimalOptimal
yivariabledual
ofvalueOptimal
1Method
19. EXAMPLE 3
Consider this problem
Max Z= 5x1 + 12x2 + 4x3
Subject to: x1 + 2x2 + x3 <= 10
2x1 - x2 + 3x3 = 8
x1, x2, x3 >= 0
20. EXAMPLE 3
Standard form
Max Z= 5x1 + 12x2 +4x3 – Mx5
Subject to: x1 + 2x2 + x3 + x4 = 10, x4 is a slack
2x1 - x2 + 3x3 + x5 = 8, x5 is artificial
x1, x2, x3, x4, x5 >= 0
21. EXAMPLE 3
The dual problem
Min W= 10y1 + 8y2
Subject to: y1 + 2y2 >= 5
2y1 - y2 >= 12
y1 + 3y2 >= 4
y1 + 0y2 >= 0 y1 >= 0, y2 is unrestricted
We now show how the optimal dual values are determined using the two methods
described before.