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Find a certain 3x3 matrixby Divergent13
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#1
Oct2004, 03:04 AM

P: 48

Hello everyone the following problem has me completely stumped, I am to find a certain 3x3 matrix D that satisfies the following equation:
[tex]ADA^{1}[/tex] = [tex]\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)[/tex] where : [tex]A = \left(\begin{array}{ccc}1&2&3\\0&1&1\\0&2&1\end{array}\right)[/tex] [tex]A^{1} = \left(\begin{array}{ccc}1&4&1\\0&1&1\\0&2&1\end{array}\right)[/tex] Heres my reasoning (or lack thereof), I know that [tex]AA^{1}[/tex] will yield the identity matrix I3, however clearly the D im looking for is WITHIN this operation, and by matrix multiplication i cannot use this fact since the order is now completely different. But what I do know is how to find the inverse of A, but what property can I use for finding a 3x3 matrix? You see this would be simpler if they were happening to look for a 3x1 matrix D where I could use row operations in gauss jordan elimination to solve for the particular values, however I did not find any examples of this problem in the book where I am given an unknown nxn matrix to find and a certain operation that it must adhere to. I could i solve this one? I have been understanding everything up to this point but i am clearly not understanding some simple rule thanks a lot for your help. 


#2
Oct2004, 03:07 AM

P: 186

Multiply both sides on the left by A inverse, then multiply both sides on the right by A...then D is on the left and you can expand out the other to find what D is.



#3
Oct2004, 02:39 PM

P: 48

Wait what do you mean by expanding out the other? What does both sides on the left mean? Like A and D? Thanks.



#4
Oct2004, 02:46 PM

P: 48

Find a certain 3x3 matrix
btw i really apologize for the stupid thread title... i was ctually testing out my TeX format and accidentally posted with a wrong name id change it if i could but i cannot!!



#5
Oct2004, 03:19 PM

P: 186

lol...its ok :)
EDIT: changed to tex [tex]ADA^{1}=\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)[/tex] [tex]A^{1}\left(ADA^{1}=\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)\right)[/tex] [tex]A^{1}ADA^{1}=A^{1}\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)[/tex] [tex]DA^{1}=A^{1}\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)[/tex] [tex]\left(DA^{1}=A^{1}\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)\right)A[/tex] [tex]DA^{1}A=A^{1}\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)A[/tex] [tex]D=A^{1}\left(\begin{array}{ccc}1&0&0\\1&0&0\\1&0&0\end{array}\right)A[/tex] 


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