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Homework Help: Matrix powers

  1. Mar 1, 2009 #1
    1. The problem statement, all variables and given/known data
    Given the matrix A= 1 2, make a rule for A^n
    0 3


    2. Relevant equations

    A^2= 1 8 , so the first and last numbers are put to the power given (i tried other powers)
    0 9

    But then what about the 8???

    Another one: A^5= 1 242
    0 243

    I need to get a general formula... it seems as though the 242 is the last number-1

    But then if I have something like B= -1 1
    -16 7
    It goes all strange

    B^2= -15 6
    -96 33

    Ok, if I make it C= a b
    c d

    Is this right for C^2: a^2+c ?
    ? d^2+c

    By the way, the ? mean I don't know
     
    Last edited: Mar 1, 2009
  2. jcsd
  3. Mar 1, 2009 #2

    Gib Z

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    Homework Helper

    Welcome to PF!

    I don't understand why you introduced the new matrix B at all? The only reason they want you to find the pattern for A is because A has an easy to spot pattern! You've pretty much found it, except you just need to know the pattern for the 2nd entry on the first row.
     
  4. Mar 1, 2009 #3

    statdad

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    Homework Helper

    Imagine starting as

    [tex] \begin{align*}
    A &=\begin{bmatrix} 1 & 2 \\ 0 & 3 \end{bmatrix} \\
    A^2 & = \begin{bmatrix} 1 & 8 \\ 0 & 9 \end{bmatrix}\\
    A^3 & = \begin{bmatrix} 1 & 26 \\ 0 & 27 \end{bmatrix}\\
    \vdots & = \vdots \\
    A^{10} & = \begin{bmatrix} 1 & 59048 \\ 0 & 59049 \end{bmatrix}
    \end{align*}
    [/tex]

    and continuing. What pattern do you see relating the entries in the first and second rows of column 2?
     
    Last edited by a moderator: Mar 1, 2009
  5. Mar 1, 2009 #4
    I see the pattern...but how would I make an equation?
     
  6. Mar 1, 2009 #5

    statdad

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    Homework Helper

    An equation can be as simple as this one (totally made up, unrelated to your matrix problem)

    [tex]
    a_1 = 2, a_2 = 3, \dots a_n = n+1
    [/tex]
     
    Last edited: Mar 1, 2009
  7. Mar 2, 2009 #6

    HallsofIvy

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    Science Advisor

    I'm not sure you really "see the pattern" since if you did it would be easy to see the equation. Presumably you are able to see that the lower right corner number of An, 3, 9, 27. 81, ... is 3n. How does the number directly above it 2, 8, 26, 80, ... relate to that?
     
  8. Mar 2, 2009 #7
    It's -1?
     
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