Find solution using graphical method
MAX z = 2x1 + x2
subject to
x1 + 2x2 <= 10
x1 + x2 <= 6
x1 - x2 <= 2
x1 - 2x2 <= 1
and x1,x2 >= 0
Solution:
Problem is
MAX `z_x` | `=` | `` | `2` | `x_1` | ` + ` | `` | `x_2` |
|
subject to |
`` | `` | `x_1` | ` + ` | `2` | `x_2` | ≤ | `10` | `` | `` | `x_1` | ` + ` | `` | `x_2` | ≤ | `6` | `` | `` | `x_1` | ` - ` | `` | `x_2` | ≤ | `2` | `` | `` | `x_1` | ` - ` | `2` | `x_2` | ≤ | `1` |
|
and `x_1,x_2 >= 0; ` |
Hint to draw constraints
1. To draw constraint `color{red}{x_1+2x_2<=10 ->(1)}`
Treat it as `color{red}{x_1+2x_2=10}`
When `x_1=0` then `x_2=?`
`=>(0)+2x_2=10`
`=>2x_2=10`
`=>x_2=(10)/(2)=5`
When `x_2=0` then `x_1=?`
`=>x_1+2(0)=10`
`=>x_1=10`
2. To draw constraint `color{green}{x_1+x_2<=6 ->(2)}`
Treat it as `color{green}{x_1+x_2=6}`
When `x_1=0` then `x_2=?`
`=>(0)+x_2=6`
`=>x_2=6`
When `x_2=0` then `x_1=?`
`=>x_1+(0)=6`
`=>x_1=6`
3. To draw constraint `color{blue}{x_1-x_2<=2 ->(3)}`
Treat it as `color{blue}{x_1-x_2=2}`
When `x_1=0` then `x_2=?`
`=>(0)-x_2=2`
`=>-x_2=2`
`=>x_2=-2`
When `x_2=0` then `x_1=?`
`=>x_1-(0)=2`
`=>x_1=2`
4. To draw constraint `color{brown}{x_1-2x_2<=1 ->(4)}`
Treat it as `color{brown}{x_1-2x_2=1}`
When `x_1=0` then `x_2=?`
`=>(0)-2x_2=1`
`=>-2x_2=1`
`=>x_2=(1)/(-2)=-0.5`
When `x_2=0` then `x_1=?`
`=>x_1-2(0)=1`
`=>x_1=1`
The value of the objective function at each of these extreme points is as follows:
Extreme Point Coordinates (`x_1`,`x_2`) | Lines through Extreme Point | Objective function value `z=2x_1 + x_2` |
`color{red}{O(0,0)}` | `color{black}{5->x_1>=0}` `color{black}{6->x_2>=0}` | `2(0)+1(0)=0` |
`color{green}{A(1,0)}` | `color{brown}{4->x_1-2x_2<=1}` `color{black}{6->x_2>=0}` | `2(1)+1(0)=2` |
`color{blue}{B(3,1)}` | `color{blue}{3->x_1-x_2<=2}` `color{brown}{4->x_1-2x_2<=1}` | `2(3)+1(1)=7` |
`color{brown}{C(4,2)}` | `color{green}{2->x_1+x_2<=6}` `color{blue}{3->x_1-x_2<=2}` | `2(4)+1(2)=10` |
`color{darkorange}{D(2,4)}` | `color{red}{1->x_1+2x_2<=10}` `color{green}{2->x_1+x_2<=6}` | `2(2)+1(4)=8` |
`color{darkviolet}{E(0,5)}` | `color{red}{1->x_1+2x_2<=10}` `color{black}{5->x_1>=0}` | `2(0)+1(5)=5` |
The maximum value of the objective function `z=10` occurs at the extreme point `(4,2)`.
Hence, the optimal solution to the given LP problem is : `x_1=4, x_2=2` and max `z=10`.
This material is intended as a summary. Use your textbook for detail explanation.
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