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Linear Algebra and its Applications
Found in: Page 437
Linear Algebra and its Applications

Linear Algebra and its Applications

Book edition 5th
Author(s) David C. Lay, Steven R. Lay and Judi J. McDonald
Pages 483 pages
ISBN 978-03219822384

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Short Answer

Question: In Exercises 15-20, write a formula for a linear functional f and specify a number d, so that \(\left( {f:d} \right)\) the hyperplane H described in the exercise.

Let H be the column space of the matrix \(B = \left( {\begin{array}{*{20}{c}}{\bf{1}}&{\bf{0}}\\{\bf{5}}&{\bf{2}}\\{ - {\bf{4}}}&{ - {\bf{4}}}\end{array}} \right)\). That is, \(H = {\bf{Col}}\,B\).(Hint: How is \({\bf{Col}}\,B\)related to Nul \({B^T}\)? See section 6.1)

The linear functional is \(f\left( {{x_1},{x_2},{x_3}} \right) = - 6{x_1} + 2{x_2} + {x_3}\) and \(d = 0\).

See the step by step solution

Step by Step Solution

Step 1: Write the matrix equation

The matrix equation can be written as follows:

\(\begin{array}{c}{B^T}{\bf{x}} = 0\\\left( {\begin{array}{*{20}{c}}1&5&{ - 4}\\0&2&{ - 4}\end{array}} \right)\left( {\begin{array}{*{20}{c}}{{x_1}}\\{{x_2}}\\{{x_3}}\end{array}} \right) = \left( {\begin{array}{*{20}{c}}0\\0\end{array}} \right)\end{array}\)

Step 2: Write the equation using matrix multiplication

The matrix equation can be simplified as shown below:

\({x_1} + 5{x_2} - 4{x_3} = 0\)


\(\begin{array}{c}2{x_2} - 4{x_3} = 0\\{x_2} = 2{x_3}\end{array}\)

Simplifying the above equations:

\(\begin{array}{c}{x_1} + 5\left( {2{x_3}} \right) - 4{x_3} = 0\\{x_1} = - 6{x_3}\end{array}\)

Step 3: Write the general solution

The general solution is:

\(\begin{array}{c}{\bf{x}} = \left( {\begin{array}{*{20}{c}}{{x_1}}\\{{x_2}}\\{{x_3}}\end{array}} \right)\\ = \left( {\begin{array}{*{20}{c}}{ - 6{x_3}}\\{2{x_3}}\\{{x_3}}\end{array}} \right)\\ = \left( {\begin{array}{*{20}{c}}{ - 6}\\2\\1\end{array}} \right){x_3}\end{array}\)

So, \(f\left( {{x_1},{x_2},{x_3}} \right) = - 6{x_1} + 2{x_2} + {x_3}\) and \(d = 0\).

Most popular questions for Math Textbooks

Let \({v_1} = \left[ {\begin{array}{*{20}{c}}{\bf{0}}\\{\bf{1}}\end{array}} \right]\), \({v_{\bf{2}}} = \left[ {\begin{array}{*{20}{c}}{\bf{1}}\\{\bf{5}}\end{array}} \right]\), \({v_{\bf{3}}} = \left[ {\begin{array}{*{20}{c}}{\bf{4}}\\{\bf{3}}\end{array}} \right]\), \({p_1} = \left[ {\begin{array}{*{20}{c}}{\bf{3}}\\{\bf{5}}\end{array}} \right]\), \({p_{\bf{2}}} = \left[ {\begin{array}{*{20}{c}}{\bf{5}}\\{\bf{1}}\end{array}} \right]\), \({p_{\bf{3}}} = \left[ {\begin{array}{*{20}{c}}{\bf{2}}\\{\bf{3}}\end{array}} \right]\), \({p_{\bf{4}}} = \left[ {\begin{array}{*{20}{c}}{ - {\bf{1}}}\\{\bf{0}}\end{array}} \right]\), \({p_{\bf{5}}} = \left[ {\begin{array}{*{20}{c}}{\bf{0}}\\{\bf{4}}\end{array}} \right]\), \({p_{\bf{6}}} = \left[ {\begin{array}{*{20}{c}}{\bf{1}}\\{\bf{2}}\end{array}} \right]\), \({p_{\bf{7}}} = \left[ {\begin{array}{*{20}{c}}{\bf{6}}\\{\bf{4}}\end{array}} \right]\) and let \(S = \left\{ {{v_1},{v_2},{v_3}} \right\}\).

  1. Show that the set is affinely independent.
  2. Find the barycentric coordinates of \({p_1}\), \({p_{\bf{2}}}\), and \({p_{\bf{3}}}\) with respect to S.
  3. On graph paper, sketch the triangle \(T\) with vertices \({v_1}\), \({v_{\bf{2}}}\), and \({v_{\bf{3}}}\), extend the sides as in Figure 8, and plot the points \({p_{\bf{4}}}\), \({p_{\bf{5}}}\), \({p_{\bf{6}}}\), and \({p_{\bf{7}}}\). Without calculating the actual values, determine the signs of the barycentric coordinates of points \({p_{\bf{4}}}\), \({p_{\bf{5}}}\), \({p_{\bf{6}}}\), and \({p_{\bf{7}}}\).

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