Elementary Linear Algebra 7th Edition

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

Chapter 3 - Determinants - 3.2 Determinants and Elementary Operations - 3.2 Exercises - Page 118: 31

Answer

$-1344$

Work Step by Step

Multiplying the first row by $-3$ and adding it to the third row. Also, adding the first row to the fourth row, we get $$\left|\begin {array}{cccc} 4&-7&9&1\\ 6&2&7&0 \\ -9&27&-30&0\\ 4&0&13&0\end {array} \right|.$$ Expanding by cofactors using the fourth column, we get $$\left|\begin {array}{cccc} 4&-7&9&1\\ 6&2&7&0 \\ -9&27&-30&0\\ 4&0&13&0\end {array} \right|=-\left|\begin {array}{cccc} 6&2&7 \\ -9&27&-30\\ 4&0&13\end {array} \right|=-(-2)\left|\begin {array}{cccc} -9& -30\\ 4& 13\end {array} \right|-27\left|\begin {array}{cccc} 6& 7\\ 4& 13\end {array} \right|=2(-117+120)-27(78-28)=6-1350=-1344.$$
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