Elementary Linear Algebra 7th Edition

Published by Cengage Learning
ISBN 10: 1-13311-087-8
ISBN 13: 978-1-13311-087-3

Chapter 4 - Vector Spaces - 4.6 Rank of a Matrix and Systems of Linear Equations - 4.6 Exercises - Page 200: 46

Answer

(a) A basis for the solution space is $$ \left[\begin{aligned}-1 \\-\frac{3}{2} \\1\\0\\0 \end{aligned}\right], \left[\begin{aligned} 2 \\ -8 \\0\\1\\0 \end{aligned}\right],\left[\begin{aligned}-1 \\ -4 \\0\\0\\1 \end{aligned}\right] .$$ (b) The dimension of the solution space is $3$.

Work Step by Step

The coefficient matrix is given by $$ \left[ \begin {array}{ccccc} 1&3&2&22&13\\ 1&0&1&-2 &1\\ 3&6&5&42&27\end {array} \right] . $$ The reduced row echelon form is $$ \left[ \begin {array}{ccccc} 1&0&1&-2&1\\ 0&1&\frac{1}{3}&8 &4\\ 0&0&0&0&0\end {array} \right] . $$ The corresponding system is $$ \begin{aligned} x_1 +x_3- 2x_4+x_5&=0\\ x_2+\frac{1}{3}x_3+8x_4+4x_5&=0\\ \end{aligned}. $$ The solution of the above system is $x_1=-r+2s-t$,$x_2=-\frac{3}{2}r-8s-4t$,$x_3=r$, $x_4=s$, $x_5=t$. This means that the solution space of $Ax = 0 $ consists of the vectors on the form $$x= \left[\begin{aligned} x_1\\ x_2\\x_3\\x_4 \\x_5 \end{aligned}\right]= \left[\begin{aligned}-r+2s-t\\-\frac{3}{2}r-8s-4t\\r\\s\\t \end{aligned}\right] = r\left[\begin{aligned}-1 \\-\frac{3}{2} \\1\\0\\0 \end{aligned}\right]+s\left[\begin{aligned} 2 \\ -8 \\0\\1\\0 \end{aligned}\right]+t\left[\begin{aligned}-1 \\ -4 \\0\\0\\1 \end{aligned}\right] .$$ (a) A basis for the solution space is $$ \left[\begin{aligned}-1 \\-\frac{3}{2} \\1\\0\\0 \end{aligned}\right], \left[\begin{aligned} 2 \\ -8 \\0\\1\\0 \end{aligned}\right],\left[\begin{aligned}-1 \\ -4 \\0\\0\\1 \end{aligned}\right] .$$ (b) The dimension of the solution space is $3$.
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