## Example ¹6. Solving a Linear Programming Problem Using a Graphical Method. |

x_{1} | - | x_{2} | ≤ | 8 | ||

3 x_{1} | - | x_{2} | ≥ | 3 | ||

2 x_{1} | + | x_{2} | ≥ | 4 |

x

Solution:

Points whose coordinates satisfy all the inequalities of the constraint system are called **a region of feasible solutions**.

It is necessary to solve each inequality of the constraint system to find the region of feasible solutions to this problem. (see step 1 - step 3)

The last two steps are necessary to get the answer.

(see step 4 - step 5)

This is a standard solution plan. If the region of feasible solutions is a point or an empty set then the solution will be shorter.

See the plan for solving this problem in pictures

By the condition of the problem: x_{1} ≥ 0 x_{2} ≥ 0.

Now we have the region of feasible solutions shown in the picture.

Step ¹1

Let's solve 1 inequality of the system of constraints.

x_{1} - x_{2} ≤ 8

We need to plot a straight line: x_{1} - x_{2} = 8

Let x_{1} =0 => - x_{2} = 8 => x_{2} = -8

Let x_{2} =0 => x_{1} = 8

Two points were found: (0, -8) and (8 ,0)

Now we can plot the straight line (1) through the found two points.

Let's go back to the inequality.

x_{1} - x_{2} ≤ 8

We need to transform the inequality so that only x_{2} is on the left side.

- x_{2} ≤ - x_{1} + 8

x_{2} ≥ x_{1} - 8

The inequality sign is ≥

Therefore, we must consider points above the straight line (1).

Let's combine this result with the previous picture.

Now we have the region of feasible solutions shown in the picture.

Step ¹2

Let's solve 2 inequality of the system of constraints.

3 x_{1} - x_{2} ≥ 3

We need to plot a straight line: 3 x_{1} - x_{2} = 3

Let x_{1} =0 => - x_{2} = 3 => x_{2} = -3

Let x_{2} =0 => 3 x_{1} = 3 => x_{1} = 1

Two points were found: (0, -3) and (1 ,0)

Now we can plot the straight line (2) through the found two points.

Let's go back to the inequality.

3 x_{1} - x_{2} ≥ 3

We need to transform the inequality so that only x_{2} is on the left side.

- x_{2} ≥ - 3 x_{1} + 3

x_{2} ≤ 3 x_{1} - 3

The inequality sign is ≤

Therefore, we must consider points below the straight line (2).

Let's combine this result with the previous picture.

Now we have the region of feasible solutions shown in the picture.

Step ¹3

Let's solve 3 inequality of the system of constraints.

2 x_{1} + x_{2} ≥ 4

We need to plot a straight line: 2 x_{1} + x_{2} = 4

Let x_{1} =0 => x_{2} = 4

Let x_{2} =0 => 2 x_{1} = 4 => x_{1} = 2

Two points were found: (0, 4) and (2 ,0)

Now we can plot the straight line (3) through the found two points.

Let's go back to the inequality.

2 x_{1} + x_{2} ≥ 4

We need to transform the inequality so that only x_{2} is on the left side.

x_{2} ≥ - 2 x_{1} + 4

The inequality sign is ≥

Therefore, we must consider points above the straight line (3).

Let's combine this result with the previous picture.

Now we have the region of feasible solutions shown in the picture.

Step ¹4

We need to plot the vector C = (3, -1), whose coordinates are the coefficients of the function F.

Step ¹5

We will move a "red" straight line perpendicular to vector C from the upper left corner to the lower right corner.

The function F has a minimum value at the point where the "red" straight line crosses the region of feasible solutions for the first time.

The function F has a maximum value at the point where the "red" straight line crosses the region of feasible solutions for the last time.

There is an assumption that the function F has a minimum value on the ray, which has its start at point A. (see picture)

Let's find the coordinates of point A.

Point A is on the straight line (2) and on the straight line (3) at the same time.

3 x_{1} | - | x_{2} | = | 3 | => | x_{1} = 7/5 | ||

2 x_{1} | + | x_{2} | = | 4 | x_{2} = 6/5 |

Let's calculate the value of the function F at point A (7/5,6/5).

F (A) = 3 * 7/5 - 1 * 6/5 = 3

The coordinates of any point of the ray can be written as follows:

x_{1} = 7/5 + 1 * t

x_{2} = 6/5 + 3 * t

t ≥ 0

Suppose that if t = 1 then we have a point B.

Let's find the coordinates of point B.

x_{1} = 7/5 + 1 * 1 = 12/5

x_{2} = 6/5 + 3 * 1 = 21/5

Let's calculate the value of the function F at point B (12/5,21/5).

F (B) = 3 * 12/5 - 1 * 21/5 = 3

F(A) = F(B).

Then we can conclude that the function F has a miniimum value at any point on the ray which has its start at point A.

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