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Linear Algebra  Determinant Properties 
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#1
Nov1108, 02:04 AM

P: 34

1. The problem statement, all variables and given/known data
1. Give an example of a 2x2 real matrix A such that A^2 = I 2. Prove that there is no real 3x3 matrix A with A^2 = I 2. Relevant equations I think these equations would apply here? det(A^x) = (detA)^x det(kA) = (k^n)detA (A being an nxn matrix) det(I) = 1 3. The attempt at a solution 1. Would I use above equations with this question? This is what I did so far; I don't know if I'm off in answering this question... I wrote: It is a 2x2 matrix, so n = 2 det(A^2) = det(I) (detA)^2 = (1^2)detI (detA)^2 = detI (and detI = 1) Therefore, detA * detA must = 1; so could I use the identity matrix itself as a matrix example for A: A = [1 0 0 1] Then, detA * detA = 1 = detI Does this make sense? Or am I not allowed to use the identity matrix here? 2. I wrote: It is a 3x3 matrix, so n = 3 det(A^2) = det(I) (detA)^2 = (1^3)detI (detA)^2 = (detI ) (detA)^2 = 1 Then, can I just say that since (detA)^2 is always positive since it is squared... therefore, (detA)^2 can never equal 1, and there is no real 3x3 matrix A with A^2 = I Thanks a lot for the help! 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution 


#3
Nov1108, 04:35 AM

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PF Gold
P: 39,682

I hate to disagree with gabbagabbahey, but you haven't answered the questions at all!



#4
Nov1108, 04:17 PM

HW Helper
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Linear Algebra  Determinant Properties
Sorry, I had a small brain fart there ...As Halls said, you are looking for an example of a matrix such that [itex]A^2=I[/itex].



#5
Nov1108, 09:24 PM

P: 34

Yeah, thanks! Oops!
Well, since I am looking for an example of a matrix A where A^2 = I, I could use the example A = [0 1 1 0] Since this squared = I But, I found this using trial and error. Is there any other way to answer this question...? It seems the question is too easy if it's just asking for an answer... is there some sort of formula or something I can use here? Thanks again. 


#6
Nov1108, 11:17 PM

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P: 25,235




#7
Nov1208, 03:07 PM

P: 34

Okay, so I could use properties from linear transformations and say:
[0 1 1 0] = I, which equals: [cos180 sin180 sin180 cos180] Then, since A^2 = [cos180 sin180 sin180 cos180] I need the square root of this, which is: [cos90 sin90 sin90 cos90] So then A = [0 1 1 0] This may sound like a stupid question, but how do I take the square root of [cos180 sin180 sin180 cos180] to obtain: [cos90 sin90 sin90 cos90] How do you take the square root of sin180, cos180 etc... Sorry I haven't done math before linear algebra in 4 years... I can't remember this stuff. Thanks!! 


#8
Nov1208, 03:15 PM

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P: 25,235

A^2(x)=A(A(x)). It just means, 'do A twice'. If you rotate by 90 twice, you get a rotation by 180. That's why I said think geometrically. Of course, you could also rotate by 90. There are two 'square roots'.



#9
Nov1208, 03:42 PM

P: 34

Ok got it. Thanks for the help!



#10
Nov1208, 03:43 PM

Math
Emeritus
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PF Gold
P: 39,682

Or write [tex]A= \left[\begin{array}{cc}a & b \\ c & d\end{array}\right][/tex] so that [tex]A^2= \left[\begin{array}{cc}a^2+ bc & ab+ bd \\ ac+ cd & bc+ d^2\end{array}\right]= \left[\begin{array}{cc}1 & 0 \\ 0 & 1\end{array}\right][/tex]
So we have [itex]a^2+ bc= 1[/itex], [itex]ab+ bd= 0[/itex], [itex]ac+ cd= 0[/itex], [itex]bc+ d^2= 1[/itex], four equations to solve for a, b, c, and d. 


#11
Dec508, 07:28 PM

P: 3

Please help me with these:
1) Prove that: nxn real matrix A is a root of f(X)= a[n].X^n+...+a[0].I, where a[n],...,a[0] are coefficients of the polynomial P(t)= det [At.I] 2) Let 5x5 real matrix A be satisfied: A^2008 = 0. Prove that: A^5=0. Thanks. 


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