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Math Help - Another matrix equation with unknown coefficients

  1. #1
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    Another matrix equation with unknown coefficients

    Got another one that is also baffling me

    Another matrix equation with unknown coefficients-49uirh.png

    After some simplification, I get

    -x+2x=0
    -x+10=1


    Nope, no solutions for x!

    5-5y=0
    5-xy=1


    Neither this one!

    The book says x=9 and y=1. Help me?
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  2. #2
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    Re: Another matrix equation with unknown coefficients

    Quote Originally Posted by SweatingBear View Post
    Got another one that is also baffling me

    \begin{bmatrix}1&5\\ 1&x \end{bmatrix}\begin{bmatrix}-x&5\\2&-y  \end{bmatrix} = \begin{bmatrix}1&0\\0&1  \end{bmatrix}

    After some simplification, I get

    -x+2x=0
    -x+10=1


    Nope, no solutions for x!

    5-5y=0
    5-xy=1


    Neither this one!

    The book says x=9 and y=1. Help me?
    As in the previous thread, the question is wrong. Either the book is full of misprints, or you are copying the questions very carelessly.

    The first matrix should be \begin{bmatrix}1&5\\ 2&x \end{bmatrix}, not \begin{bmatrix}1&5\\ 1&x \end{bmatrix}.
    Last edited by Opalg; December 5th 2011 at 11:48 AM.
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  3. #3
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    Re: Another matrix equation with unknown coefficients

    Quote Originally Posted by Opalg View Post
    As in the previous thread, the question is wrong. Either the book is full of misprints, or you are copying the questions very carelessly.

    The first matrix should be \begin{bmatrix}1&5\\ 2&x \end{bmatrix}, not \begin{bmatrix}1&5\\ 1&x \end{bmatrix}.
    Misprints. Thx.
    By the way how do you know how they are supposed to be?
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  4. #4
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    Re: Another matrix equation with unknown coefficients

    Quote Originally Posted by SweatingBear View Post
    By the way how do you know how they are supposed to be?
    These are inverse matrices, that is, two matrices whose product is the identity matrix \begin{bmatrix}1&0\\0&1 \end{bmatrix}. There is a formula for the inverse of a matrix, which makes it easy to see what the question should be asking.
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