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Section 1.7 Exercises

  1. Reduce the following matrices into row-echelon form.
    \begin{align*} \begin{pmatrix} 1 \amp 2 \amp 3 \amp 4\\ 2 \amp 1 \amp 4 \amp 5\\ 1 \amp 5 \amp 5 \amp 7\\ 8\amp 1 \amp 14 \amp 17 \end{pmatrix}, \amp \begin{pmatrix} 1 \amp 2 \amp -1 \amp 1\\ 2 \amp 3 \amp 4 \amp 5\\ 1 \amp 4 \amp -13 \amp -5 \end{pmatrix}\\ \begin{pmatrix} 2 \amp 3 \amp 1 \amp 0 \amp 4\\ 3 \amp 1 \amp 2 \amp -1\amp 1\\ 4 \amp -1 \amp 3 \amp -2\amp -2\\ 5 \amp 4 \amp 3 \amp -1 \amp 5 \end{pmatrix},\amp \begin{pmatrix} -1 \amp 1 \amp 2\\ 3 \amp -1 \amp 1\\ 1\amp 3\amp 4 \end{pmatrix} \end{align*}
  2. Find the sequence of row operations that converts the matrix.
    \begin{equation*} \begin{bmatrix} a_1+b_1 \amp a_2+b_2 \amp a_3+b_3\\ a_1+c_1 \amp a_2+c_2 \amp a_3+c_3\\ b_1+c_1 \amp b_2+c_2 \amp b_3+c_3 \end{bmatrix} \text{ to } \begin{bmatrix} a_1\amp a_2 \amp a_3\\ b_1 \amp b_2 \amp b_3\\ c_1 \amp c_2 \amp c_3 \end{bmatrix} \end{equation*}
  3. Solve the following systems of linear equations by Gaussian elimination method.
    1. \begin{equation*} \begin{array}{ccc} x+2y+3z \amp = \amp 2 \\ 3x+7y+9z\amp = \amp 0 \\ x+4y+z\amp = \amp 2 \\ \end{array} \end{equation*}
    2. \begin{equation*} \begin{array}{ccc} x-2z\amp = \amp 0 \\ y+3z\amp = \amp 0 \\ 4x+y+2z\amp = \amp 0 \\ \end{array} \end{equation*}
    3. \begin{equation*} \begin{array}{ccc} x+y+2z \amp = \amp 4 \\ x-y+z \amp = \amp 2 \\ \end{array} \end{equation*}
    4. \begin{equation*} \begin{array}{ccc} x+y+2z \amp = \amp 0 \\ x-y+z \amp = \amp 0 \\ \end{array} \end{equation*}
    5. \begin{equation*} \begin{array}{ccc} x+y\amp = \amp -2\\ 2x+3y+z\amp = \amp 1 \\ 3x+2y+z\amp = \amp 3\\ \end{array} \end{equation*}
    6. \begin{equation*} \begin{array}{ccc} x-y+2z \amp = \amp 4\\ 3x+y+z \amp = \amp -2\\ 2x-3y+3z \amp = \amp 0 \end{array} \end{equation*}
  4. Find the rank of the following matrices:
    \begin{equation*} A=\begin{pmatrix} 1 \amp 2 \amp 3\\2\amp 1 \amp -1\\1 \amp -1 \amp 2\\5 \amp 4\amp -5 \end{pmatrix}, B= \begin{pmatrix} 2 \amp 1 \amp 5\amp -1 \amp \\-1 \amp 2\amp 5\amp 3\\3 \amp 2\amp 9\amp -1 \end{pmatrix}, C= \begin{pmatrix} 1 \amp 1 \amp 1 \\a \amp b \amp c\\a^2 \amp b^2\amp c^2 \end{pmatrix} \end{equation*}
  5. Find all values of \(k\) such that the rank of the matrix \(\begin{pmatrix} k \amp -1 \amp 0\amp 0 \\0 \amp k \amp -1\amp 0\\0\amp 0 \amp k\amp -1\\-6 \amp 11\amp -6 \amp 1 \end{pmatrix}\) is 3.
  6. Find all values of \(k\) such that the rank of the matrix \(\begin{bmatrix} k+3 \amp 1 \amp -2\\ 3 \amp -2 \amp k\\-k \amp -3 \amp 3 \end{bmatrix}\) is 3.
  7. For the following system of equations write the solution in the form of \(X_0+S_h\) where \(X_0\) is a solution of non homogeneous system and \(S_h\) is the set of solutions of the corresponding homogeneous system
    (i) \(3x-4y=1\)
    (ii) \(-3x + 6 y + -11z = 4; 3x - 4y + 6z = -3\)
    (iii) \(x+y+z-w = 1; 3x+6z-6w = 6; -y+z-w = 1 \)
    (iv) \(x+2y+4z = 3; 2x+4y+z = 3 \)
  8. Balance the following photosynthesis reaction
    \begin{equation*} CO_2 + H_2O \to C_6H_{12}O_6 + O_2\text{.} \end{equation*}
    Here \(C_6H_{12}O_6\) is glucose.
  9. The augmented matrix of a linear system has the form
    \begin{equation*} \begin{bmatrix} 1 \amp 2\amp -1 \amp a \\ 2 \amp 3\amp -2 \amp b\\ -1 \amp -1\amp 1 \amp c \end{bmatrix} \end{equation*}
    (a) Determine the values of a, b, and c for which the linear system is consistent.
    (b) Determine the values of a, b, and c for which the linear system is inconsistent.
    (c) When it is consistent, does the linear system have a unique solution or infinitely many solutions?
    (d) Give a specific consistent linear system and find one particular solution.
  10. Solve the following system of equations for \(x\) and \(y\text{.}\)
    \begin{equation*} x^2 + xy =y^2 = 1, \quad 2x^2 - xy + 3y^2 = 13,\quad x^2 + 3xy + 2y^2 = 0\text{.} \end{equation*}
  11. Find a polynomial of the form \(y = a_0 +a_1 x+a_2 x^2 +a_3 x^3\) which passes through the points \((-3, -2), (-1, 2), (1, 5), (2, 1)\text{.}\)
  12. Find the values of \(a\) and \(b\) for which the following system is consistent. Also find the complete solution when \(a = b = 2\text{.}\)
    \begin{equation*} x+y-z+w = 1;\quad ax + y + z + w = b;\quad 3x + 2y + aw = 1 + a\text{.} \end{equation*}
  13. The following table lists the number of milligrams of vitamin A, vitamin B, vitamin C, and niacin contained in 1 g of four different foods. A dietitian wants to prepare a meal that provides 250 mg of vitamin A, 300 mg of vitamin B, 400 mg of vitamin C, and 70 mg of niacin. Determine how many grams of each food must be included, and describe any limitations on the quantities of each food that can be used
    \begin{equation*} \begin{array}{ccccc} Nutrients \amp Group~1 \amp Group~2 \amp Group~3 \amp Group \\ Vitamin~A \amp 20 \amp 30 \amp 40 \amp 10\\ Vitamin~B \amp 40 \amp 20 \amp 35 \amp 20\\ Vitamin~C \amp 50 \amp 40 \amp 10 \amp 30\\ Niacin \amp 5 \amp 5 \amp 10 \amp 5 \end{array} \end{equation*}
  14. Let \(A\) be the coefficient matrix of the following homogeneous system of \(n \) equations in \(n\) unknowns:
    \begin{equation*} \begin{array}{ccc} (1-n)x_1 + x_2 + \cdots + x_n \amp=\amp 0\\ x_1 + (1- n)x_2 + \cdots + x_n \amp=\amp 0\\ \vdots\amp\amp \vdots\\ x_1 + x_2 + \cdots + (1-n)x_n \amp=\amp 0. \end{array} \end{equation*}
    Find the reduced row-echelon form of \(A\) and hence, or otherwise, prove that the solution of the above system is \(x_1 = x_2 = \ldots = x_n\) , with \(x_n\) arbitrary.
  15. For which rational numbers \(a\) does the following system have (i) no solutions (ii) exactly one solution (iii) infinitely many solutions?
    \begin{equation*} x + 2y-3z = 4;\quad 3x-y + 5z = 2;\quad 4x + y + (a^2-14)z = a + 2\text{.} \end{equation*}
  16. If \((\alpha_1 ,\ldots,\alpha_n)\) and \((\beta_1 ,\ldots,\beta_n)\) are solutions of a system of linear equations, prove that
    \begin{equation*} ((1-t)\alpha_1+t\beta_1 , \ldots, (1-t)\alpha_n+t\beta_n) \end{equation*}
    is also a solution.
  17. Solve the system \(AX=b\) using Doolittle method:
    (i) \(A=\begin{pmatrix} 25\amp 5\amp 1\\64\amp 8\amp 1\\144\amp 12\amp 1 \end{pmatrix}\) and \(b=\begin{pmatrix}3\\1\\-2\end{pmatrix}\)
    (ii) \(A= \begin{pmatrix}1\amp 1\amp 1\\3\amp 1\amp -3\\1\amp -2\amp -5 \end{pmatrix} \) and \(b=\begin{pmatrix}-3\\7\\2\end{pmatrix}\text{.}\)
  18. Solve the system \(AX=b\) using Crout’s method
    (i) \(A=\begin{pmatrix} 2\amp -4\amp 3\\4\amp 1\amp -6\\5\amp 8\amp -4 \end{pmatrix}\) and \(b=\begin{pmatrix}2\\-1\\5\end{pmatrix}\)
    (ii) \(A= \begin{pmatrix} 1\amp 0\amp 5\amp 0\\-1\amp 4\amp 1\amp 0\\3\amp 0\amp 4\amp 1\\-2\amp 1\amp 1\amp 3 \end{pmatrix}\) and \(b=\begin{pmatrix}1\\4\\3\\5\end{pmatrix}\)