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Math Help - Matrices: Problem

  1. #1
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    Lightbulb Matrices: Problem

    Hello All,

    I have been having much difficulty understanding matrices and a group of us have become desperate to find some comprehension of this problem. We have come up with an answer for c, but still not sure if it's correct. If you could please be of assistance it would be greatly appreciated!

    A company makes three types of product (A,B, and C) with specifications below:

    Product | A | B | C | Total Allowed
    Weight 20 20 60 220 lbs
    Volume 20 15 20 125 cu ft

    a) Write a system of equations that could be used to determine what combination of products should be shipped.

    b) Write the augmented matrix that matches the system.

    c) Perform row operations to obtain a reduced matrix.

    d) Interpret the matrix to determine the quantities to be shipped.
    Last edited by seananigans; October 9th 2012 at 09:53 PM.
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  2. #2
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    Re: Matrices: Problem

    Hey seananigans.

    Are there constraints for the total volume and the total weight?
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  3. #3
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    Re: Matrices: Problem

    Hello Chiro,

    I believe it would be as follows

    Total Allowed Weight: 220lbs

    Total Allowed Volume: 125 cu ft
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  4. #4
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    Re: Matrices: Problem

    Hello, seananigans1


    A company makes three types of product (A,B, and C) with specifications below:

    . . \begin{array}{|c||c|c|c||c|} \hline \text{Product} & A & B & C & \text{Maximum} \\ \hline \text{Weight} & 20 & 20 & 60 & 220\text{ lbs} \\ \text{Volume} & 20 & 15 & 20 & 125\text{ cu ft} \\ \hline\end{array}


    a) Write a system of equations that could be used
    . . to determine what combination of products should be shipped.

    \text{Let }\:\begin{Bmatrix} A &=& \text{units of A to be shipped} \\ B &=& \text{units of B to be shipped} \\ C &=& \text{units of C to be shipped} \end{Bmatrix} \quad A,B,C\:\ge\:0

    Then we have: . \begin{Bmatrix}20A + 20B + 60C \:\le\:220 & \Rightarrow & A + B + 3C \:\le\:11 \\ 20A + 15B + 20C \:\le\:125 & \Rightarrow & 4A + 3B + 4C \:\le\:25 \end{Bmatrix}



    b) Write the augmented matrix that matches the system.

    \left|\begin{array}{ccc|c} 1&1&3&11 \\ 4&3&4&25 \end{array}\right|



    c) Perform row operations to obtain a reduced matrix.

    We have: . \left|\begin{array}{ccc|c} 1&1&3&11 \\ 4&3&4&25 \end{array}\right|

    \begin{array}{c} \\ R_2-4R_1\end{array}\:\left|\begin{array}{ccc|c}1&1&3&1  1 \\ 0&\text{-}1&\text{-}8&\text{-}19 \end{array}\right|

    \begin{array}{c}R_1+R_2 \\ \text{-}1R_2 \end{array}\:\left|\begin{array}{ccc|c} 1&0&\text{-}5& \text{-}8 \\ 0&1&8&19 \end{array}\right|



    d) Interpret the matrix to determine the quantities to be shipped.

    We have: . \begin{Bmatrix}A -5C  \:=\:\text{-}8 & \Rightarrow & A &=& 5C - 8 \\ B+8C \:=\:19 & \Rightarrow & B &=& \text{-}8C + 19 \\ && C &=& C\end{Bmatrix}

    \text{On the right, replace }C\text{ with a parameter }t:\;\begin{Bmatrix} A &=& 5t - 8 \\ B &=& \text{-}8t+19 \\ C &=& t \end{Bmatrix}


    We have parametric equations for A,B,C for any value of t.

    But A,B,C must be nonnegative.

    A\,\ge\,0 \quad\Rightarrow\quad 5t-8\:\ge\:0 \quad\Rightarrow\quad t \:\ge\:\tfrac{8}{5}

    B\,\ge\,0 \quad\Rightarrow\quad \text{-}8t + 19 \:\ge\:0 \quad\Rightarrow\quad t \:\le\;\tfrac{19}{8}


    We have: . 1\tfrac{3}{5}\:\le\:t\:\le\:2\tfrac{3}{8} \quad\Rightarrow\quad t \,=\,2

    Therefore: . \begin{Bmatrix} A&=& 2 \\ B&=&3 \\ C&=&2 \end{Bmatrix}
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  5. #5
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    Re: Matrices: Problem

    Sorry, I was thinking this was some sort of operations research problem where you had packing constraints (like a trucks storage or something along those lines)
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  6. #6
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    Re: Matrices: Problem

    Thank you very much Soroban! This is a load of pressure off my chest, as well as our groups - I do have a question regarding both part A, if you don't mind.
    I'm having difficulty understanding why and how the initial matrix is rewritten as is in part A.

    ----> Matrices: Problem-screen-shot-2012-10-10-11.37.01-pm.png
    Last edited by seananigans; October 10th 2012 at 08:40 PM.
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