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Section 3.8 Row Operations via Matrix Multiplication: In-Class Practice

Worksheet 3.8.1 Worksheet

  1. Match the matrix with the row operation.
    1. \(\displaystyle \lmatrix{ccc} 0\amp 0\amp 1\\0\amp 1\amp 0\\1\amp 0\amp 0 \rmatrix\)
    2. \(\displaystyle \lmatrix{ccc} 1\amp 0\amp 0\\-2\amp 1\amp 0\\0\amp 0\amp 1 \rmatrix \)
    3. \(\displaystyle \lmatrix{rrr} -2\amp 0\amp 0\\0\amp 1\amp 0\\0\amp 0\amp 1 \rmatrix \)
    4. \(\displaystyle \lmatrix{ccc} 1\amp -2\amp 0\\0\amp 1\amp 0\\0\amp 0\amp 1 \rmatrix \)
    5. \(\displaystyle \lmatrix{ccc} 1\amp 0\amp 0\\1\amp -2\amp 0\\0\amp 0\amp 1 \rmatrix \)
    1. \(\displaystyle -2R_1+R_2 \rightarrow R_2\)
    2. \(\displaystyle R_1-2R_2 \rightarrow R_2\)
    3. \(\displaystyle R_1\leftrightarrow R_3 \)
    4. \(\displaystyle -2R_1 \rightarrow R_1 \)
    5. \(\displaystyle R_1 - 2 R_2 \rightarrow R_1 \)
  2. Consider the system of equations
    \begin{equation*} \begin{array}{rrcrcrcr} R_1:\, \amp 3x \amp - \amp y \amp + \amp 2z \amp = \amp -8\\ R_2:\, \amp -x\amp +\amp 2y \amp + \amp 11z\amp =\amp 21\\ R_3:\, \amp x\amp -\amp 7y\amp +\amp z\amp =\amp -29 \end{array} \end{equation*}
    1. Write the matrix \(A\) that corresponds to the system above.
    2. Swap rows \(R_1\) and \(R_2\) and write down the new system of equations and the elementary matrix \(E_1=E_{12}\) that performs the swap.
    3. Continue to perform row operations on the system until it is in row-echelon form. At each step write down and keep track of the corresponding elementary matrix; that is, \(E_1=E_{12}\) (elementary matrix corresponding to your first row operation), \(E_2\text{,...,}\)\(E_m\) (elementary matrix corresponding to the last row operation). Note each group member should do this on their own (it’s ok if you do it in different order - just check each person all verify in part (e)).
    4. Multiply the elementary matrices together in reverse order \(E=E_m\cdots E_2E_1.\)
    5. Perform the multiplication \(EA\) and verify it corresponds to the row-echelon form you found in part Item 2.c.
  3. Let \(E\in \R^{3 \times 3}\) be the matrix that swaps rows \(1\) and \(3\text{.}\) Write down the matrix \(E_{13}\) and show that \(E_{13}^{-1}=E_{13}\text{.}\)
  4. Let \(E \in \R^{3 \times 3}\) be the matrix that performs a scalar multiplication of row \(R_2\) by \(5\text{,}\) \(E_{(5)22}\text{.}\) Write down \(E_{(5)22}\) and make a guess as to its inverse. Check your answer and keep guessing until you find the inverse. Do you notice a relationship between \(E_{(5)22}\) and \(E_{(5)22}^{-1}\text{?}\)
  5. Let \(E\in \R^{3 \times 3}\) be the matrix that performs the operation \(E_{(2)13}\text{.}\) Write down the matrix \(E_{(2)13}\) and find its inverse using a guess and check method. Do you notice a relationship between \(E_{(2)13}\) and \(E_{(2)13}^{-1}\text{?}\)