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Math Help - Matrices det (a)=0

  1. #1
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    Matrices det (a)=0

    The question is that i shall find all numbers for a so that det (A)=0
    i basicly dont know what trick i shall use!
    edit: i did start to det it and set it to zero but i get wrong
    Attached Thumbnails Attached Thumbnails Matrices det (a)=0-image.jpg  
    Last edited by Petrus; November 29th 2012 at 10:39 AM.
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  2. #2
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    Re: Matrices det (a)=0

    First find the determinant using cofactor expansion method.
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    Re: Matrices det (a)=0

    yes i did it.. i get -2a^2+10a-20 and then i set it to zero and get the answer
    a=25
    a=-10
    the answe shall be 3 and 2 but idk what i do wrong =S
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    Re: Matrices det (a)=0

    Quote Originally Posted by Petrus View Post
    The question is that i shall find all numbers for a so that det (A)=0
    i basicly dont know what trick i shall use!
    edit: i did start to det it and set it to zero but i get wrong
    Expand these and solve for a.

    2\left| {\begin{array}{rr}   a & 1  \\   2 & {2a + 2}  \\ \end{array} } \right| - \left| {\begin{array}{rr}   { - 2} & 1  \\   {2a} & 2  \\ \end{array} } \right| + 3\left| {\begin{array}{rr}   { - 2} & a  \\   a & 2  \\ \end{array} } \right| = 0
    Thanks from Petrus
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    Re: Matrices det (a)=0

    Quote Originally Posted by Plato View Post
    Expand these and solve for a.

    2\left| {\begin{array}{rr}   a & 1  \\   2 & {2a + 2}  \\ \end{array} } \right| - \left| {\begin{array}{rr}   { - 2} & 1  \\   {2a} & 2  \\ \end{array} } \right| + 3\left| {\begin{array}{rr}   { - 2} & a  \\   a & 2  \\ \end{array} } \right| = 0
    so im basicly pretty new on matrice :P ik how to expaned but why did u do it? and why cant we use sorrow rule or what the name to find det on 3x3 matrice?
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    Re: Matrices det (a)=0

    Quote Originally Posted by Petrus View Post
    so im basicly pretty new on matrice :P ik how to expaned but why did u do it? and why cant we use sorrow rule or what the name to find det on 3x3 matrice?
    I have no idea what the meaning of that may be.

    You should get -2a^2+10a-12 and answers 2,~3.
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