# Converting linear programming problems into standard form

I have the following linear programming problem:

1. Convert the following problems to standard form: \begin{align} \text{a)}&\text{minimize}&x+2y+3z\\ & \text{subject to}&2\le x+y\le 3\\ & &4\le x+z \le 5\\ & &x\ge 0, \,\,\,y\ge0\,\,\,z\ge0.\\ \\ \text{b)}&\text{minimize}&x+y+z\\ & \text{subject to}&2\le x+y\le 3\\ & &x\ge 1, \,\,\,y\ge2\,\,\,z\ge1.\\ \end{align}

Here is my attempted solution:

$(1a)$ First I note that: $x+y \ge 2, x+y \leq 3, x+y\ge 4, x+y \leq 5$ with $x,y,z \ge 0$. I transform the equation into standard form by selecting two surplus and two slack variables $a, b, c, d$. So I get:

$$\text{minimize} \;\;\; x+2y+3z$$ $$\text{subject to} \;\;\; x+y + a= 3$$ $$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; x+y - b= 2$$ $$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; x+y + c= 5$$ $$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; x+y - d= 4$$ $$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; x \ge 0, \;\; y \ge 0, \;\; z \ge 0, \;\; a \ge 0, \;\; b \ge 0, \;\; c \ge 0, \;\; d \ge 0 \;\;.$$

$(1b)$ I do a change of variables by setting $a = x-1, b=y-2, c=z-1$. Then I formulate the problem in terms of variables $a, b$ and $c$ :

$$\text{minimize} \;\;\; a+b+c+4$$ $$\text{subject to} \;\;\; a+2b + 3c= 2$$ $$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; a \ge 0, \;\; b \ge 0, \;\; c \ge 0, \;\;.$$

Now this problem is equivalent with the original problem and by solving the optimal values for $a, b$ and $c$ I can solve the values for $x, y$ and $z$.

I would appreciate if someone can correct my mistakes if any exist :) I'm new with linear and nonlinear programming and want verification whether I have understood the basic concepts or not :)

Thank you!

• Please, one problem per question only. – Ataraxia Aug 23 '13 at 8:54
• Oh, sorry about that. I tried to optimize :) – jjepsuomi Aug 23 '13 at 9:03
• Can't fault a person for trying to optimize :) It's discouraged for a number of reasons. It's acceptable if all the problems are related in some way (and it's preferable if you tie them all together with a single question). – Ataraxia Aug 23 '13 at 9:09
• aah, I get it. Thank you :) I will keep that in mind. Should I edit my question then to include only one and make two separate ones also? – jjepsuomi Aug 23 '13 at 9:18
• Yea, that would be best. And you're quite welcome. – Ataraxia Aug 23 '13 at 9:23

@jjepsuomi $(1a)$: I think your third and fourth lines of constraints should be something like $x + z + c=5$ and $x+z-d=4$ instead.