Determinants Question

**egshih** · Jul 30th 2009, 10:26 PM

Hi I ran across this problem and could not figure it out. Could someone please give me an explanation? I feel like I have something close but no cigar.

Compute the following determinants by using row and/or column reduction

$\begin{bmatrix}3&-6&15\\-4&-5&-2\\2&7&3\end{bmatrix}$

Then, I applied the following row operations.
R2->R2+2R3
R3->3R3-2R1
I also factored out the 3 from the top row.

3 $\begin{bmatrix}1&-2&5\\0&9&4\\0&33&-21\end{bmatrix}$

Then, R3->R3-33/9R2

3 $\begin{bmatrix}1&-2&5\\0&9&4\\0&0&\frac{-107}{3}\end{bmatrix}$

Det=3*1*9*-107/3=-963

However, the answer should be -321.

**Opalg** · Jul 30th 2009, 11:57 PM

Originally Posted by egshih

Hi I ran across this problem and could not figure it out. Could someone please give me an explanation? I feel like I have something close but no cigar.

Compute the following determinants by using row and/or column reduction

$\begin{vmatrix}3&-6&15\\-4&-5&-2\\2&7&3\end{vmatrix}$

Then, I applied the following row operations.
R2->R2+2R3
R3->3R3-2R1 Here's where it goes wrong. When you multiply R3 by 3 you multiply the determinant by 3. That's why you end up with 3 times the correct answer.
I also factored out the 3 from the top row.

3 $\begin{vmatrix}1&-2&5\\0&9&4\\0&33&-21\end{vmatrix}$

Then, R3->R3-33/9R2

3 $\begin{vmatrix}1&-2&5\\0&9&4\\0&0&\frac{-107}{3}\end{vmatrix}$

Det=3*1*9*-107/3=-963

However, the answer should be -321.

Also, in LaTeX you should use vmatrix rather than bmatrix to get a determinant.

**Krizalid** · Jul 31st 2009, 06:16 AM

$\left| \begin{matrix} \phantom{-}3 & -6 & \phantom{-}15 \\ -4 & -5 & -2 \\ \phantom{-}2 & \phantom{-}7 & \phantom{-}3 \end{matrix} \right|=3\left| \begin{matrix} \phantom{-}1 & -2 & \phantom{-}5 \\ -4 & -5 & -2 \\ \phantom{-}2 & \phantom{-}7 & \phantom{-}3 \end{matrix} \right|,$ now the determinant equals $3\left| \begin{matrix} 1 & -2 & \phantom{-}5 \\ 0 & \phantom{-}9 & \phantom{-}4 \\ 0 & \phantom{-}11 & -7 \end{matrix} \right|=3\left| \begin{matrix} 9 & \phantom{-}4 \\ 11 & -7 \end{matrix} \right|=-321.$

Don't waste your time producing more zeros.

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