Question #200609

(5.1)Find det(−2A) and compare it to det(A) for

A= 1 1

3 -1


(5.2)Find det(−2A) and compare it to det(A) for


-2 1 3

A = 1 4 5

2 3 1



1
Expert's answer
2021-05-31T19:17:02-0400

Solution:

(5.1):

A=[1131]A=1131=1(1)3(1)=4 ...(i)A=\begin{bmatrix}1&1\\ \:\:3&-1\end{bmatrix} \\|A|=\begin{vmatrix}1&1\\ \:\:3&-1\end{vmatrix}=1(-1)-3(1)=-4\ ...(i)

2A=2[1131]=[2262]2A=2262=2(2)(2)(6)=412=16-2A=-2\begin{bmatrix}1&1\\ \:\:3&-1\end{bmatrix}=\begin{bmatrix}-2&-2\\ \:\:-6&2\end{bmatrix} \\|-2A|=\begin{vmatrix}-2&-2\\ \:\:-6&2\end{vmatrix}=-2(2)-(-2)(-6)=-4-12=-16

Now, 2A=16=4(4)=4A|-2A|=-16=-4(4)=4A [Using (i)]

(5.2):

A=[213145231]A=213145231=2det(4531)1det(1521)+3det(1423)A=\begin{bmatrix}-2&1&3\\ \:\:1&4&5\\ \:\:2&3&1\end{bmatrix} \\|A|=\begin{vmatrix}-2&1&3\\ \:\:1&4&5\\ \:\:2&3&1\end{vmatrix} \\=-2\cdot \det \begin{pmatrix}4&5\\ 3&1\end{pmatrix}-1\cdot \det \begin{pmatrix}1&5\\ 2&1\end{pmatrix}+3\cdot \det \begin{pmatrix}1&4\\ 2&3\end{pmatrix}

=2(11)1(9)+3(5)=16=-2\left(-11\right)-1\cdot \left(-9\right)+3\left(-5\right)=16 ...(i)

Now, 2A=2[213145231]-2A=-2\begin{bmatrix}-2&1&3\\ \:\:1&4&5\\ \:\:2&3&1\end{bmatrix}

=[4262810462]=\begin{bmatrix}4&-2&-6\\ -2&-8&-10\\ -4&-6&-2\end{bmatrix}

2A=4det(81062)(2)det(21042)6det(2846)|-2A|=4\cdot \det \begin{pmatrix}-8&-10\\ -6&-2\end{pmatrix}-\left(-2\right)\det \begin{pmatrix}-2&-10\\ -4&-2\end{pmatrix}-6\cdot \det \begin{pmatrix}-2&-8\\ -4&-6\end{pmatrix}

=4(44)(2)(36)6(20)=128=16(8)=8A  [using (i)]=4\left(-44\right)-\left(-2\right)\left(-36\right)-6\left(-20\right)=-128 \\=16(-8)=-8|A| \ \ [\text{using (i)}]


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