Activity1.4.2

  1. Shown below are three augmented matrices in reduced row echelon form.

    \begin{equation*} \left[ \begin{array}{rrr|r} 1 \amp 0 \amp 0 \amp 3 \\ 0 \amp 1 \amp 0 \amp 0 \\ 0 \amp 0 \amp 1 \amp -2 \\ 0 \amp 0 \amp 0 \amp 0 \\ \end{array} \right] \end{equation*}

    \begin{equation*} \left[ \begin{array}{rrr|r} 1 \amp 0 \amp 2 \amp 3 \\ 0 \amp 1 \amp -1 \amp 0 \\ 0 \amp 0 \amp 0 \amp 0 \\ 0 \amp 0 \amp 0 \amp 0 \\ \end{array} \right] \end{equation*}

    \begin{equation*} \left[ \begin{array}{rrr|r} 1 \amp 0 \amp 2 \amp 0 \\ 0 \amp 1 \amp -1 \amp 0 \\ 0 \amp 0 \amp 0 \amp 1 \\ 0 \amp 0 \amp 0 \amp 0 \\ \end{array} \right] \end{equation*}

    For each matrix, identify the pivot positions and determine if the corresponding linear system is consistent. Explain how the location of the pivots determine consistency or inconsistency.

  2. Each of these augmented matrices above has a row in which each entry is zero. What, if anything, does the presence of such a row tell us about the consistency of the corresponding linear system?

  3. Give an example of a \(3\times5\) augmented matrix in reduced row echelon form that represents a consistent system. Indicate the pivot positions in your matrix and explain why these pivot positions guarantee a consistent system.

  4. Give an example of a \(3\times5\) augmented matrix in reduced row echelon form that represents an inconsistent system. Indicate the pivot positions in your matrix and explain why these pivot positions guarantee an inconsistent system.

  5. Write the reduced row echelon form of the coefficient matrix of the corresponding linear system in ItemĀ d? (Remember that PropositionĀ 1.3.1 says that the reduced row echelon form of the coefficient matrix simply consists of the first four columns of the augmented matrix.) What do you notice about the pivot positions in this coefficient matrix?

  6. Suppose we have a linear system for which the coefficient matrix has the following reduced row echelon form.

    \begin{equation*} \left[ \begin{array}{rrrrr} 1 \amp 0 \amp 0 \amp 0 \amp -1 \\ 0 \amp 1 \amp 0 \amp 0 \amp 2 \\ 0 \amp 0 \amp 1 \amp 0 \amp 0 \\ 0 \amp 0 \amp 0 \amp 1 \amp -3 \\ \end{array} \right] \end{equation*}

    What can you say about the consistency of the linear system?

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