Solve linear equation system with Gauss












1












$begingroup$


I have the following matrix and have to see if it has solutions depending on $a$. My solution:



$M=
left[ {begin{array}{cc}
a & a^2 &| &1 \
-1 & -1& | & -a \
1 & a & | & a
end{array} } right]
$



My attemp was:
Changing first with third line



$=
left[ {begin{array}{cc}
1 & a & | & a \
-1 & -1& | & -a \
a & a^2 &| &1 \
end{array} } right]
$



Add the first row to the second one



$=
left[ {begin{array}{cc}
1 & a & | & a \
0 & a-1& | & 0 \
a & a^2 &| &1 \
end{array} } right]
$



Add the $-a$ of the first line to the third line



$=
left[ {begin{array}{cc}
1 & a & | & a \
0 & a-1& | & 0 \
0 & 0 &| &1-a^2 \
end{array} } right]
$



From $0=1-a^2$there we can obtain that the LES has a solution if $a=+/-1$



Is this a valid solution to the problem and is there a free variable?










share|cite|improve this question









$endgroup$

















    1












    $begingroup$


    I have the following matrix and have to see if it has solutions depending on $a$. My solution:



    $M=
    left[ {begin{array}{cc}
    a & a^2 &| &1 \
    -1 & -1& | & -a \
    1 & a & | & a
    end{array} } right]
    $



    My attemp was:
    Changing first with third line



    $=
    left[ {begin{array}{cc}
    1 & a & | & a \
    -1 & -1& | & -a \
    a & a^2 &| &1 \
    end{array} } right]
    $



    Add the first row to the second one



    $=
    left[ {begin{array}{cc}
    1 & a & | & a \
    0 & a-1& | & 0 \
    a & a^2 &| &1 \
    end{array} } right]
    $



    Add the $-a$ of the first line to the third line



    $=
    left[ {begin{array}{cc}
    1 & a & | & a \
    0 & a-1& | & 0 \
    0 & 0 &| &1-a^2 \
    end{array} } right]
    $



    From $0=1-a^2$there we can obtain that the LES has a solution if $a=+/-1$



    Is this a valid solution to the problem and is there a free variable?










    share|cite|improve this question









    $endgroup$















      1












      1








      1





      $begingroup$


      I have the following matrix and have to see if it has solutions depending on $a$. My solution:



      $M=
      left[ {begin{array}{cc}
      a & a^2 &| &1 \
      -1 & -1& | & -a \
      1 & a & | & a
      end{array} } right]
      $



      My attemp was:
      Changing first with third line



      $=
      left[ {begin{array}{cc}
      1 & a & | & a \
      -1 & -1& | & -a \
      a & a^2 &| &1 \
      end{array} } right]
      $



      Add the first row to the second one



      $=
      left[ {begin{array}{cc}
      1 & a & | & a \
      0 & a-1& | & 0 \
      a & a^2 &| &1 \
      end{array} } right]
      $



      Add the $-a$ of the first line to the third line



      $=
      left[ {begin{array}{cc}
      1 & a & | & a \
      0 & a-1& | & 0 \
      0 & 0 &| &1-a^2 \
      end{array} } right]
      $



      From $0=1-a^2$there we can obtain that the LES has a solution if $a=+/-1$



      Is this a valid solution to the problem and is there a free variable?










      share|cite|improve this question









      $endgroup$




      I have the following matrix and have to see if it has solutions depending on $a$. My solution:



      $M=
      left[ {begin{array}{cc}
      a & a^2 &| &1 \
      -1 & -1& | & -a \
      1 & a & | & a
      end{array} } right]
      $



      My attemp was:
      Changing first with third line



      $=
      left[ {begin{array}{cc}
      1 & a & | & a \
      -1 & -1& | & -a \
      a & a^2 &| &1 \
      end{array} } right]
      $



      Add the first row to the second one



      $=
      left[ {begin{array}{cc}
      1 & a & | & a \
      0 & a-1& | & 0 \
      a & a^2 &| &1 \
      end{array} } right]
      $



      Add the $-a$ of the first line to the third line



      $=
      left[ {begin{array}{cc}
      1 & a & | & a \
      0 & a-1& | & 0 \
      0 & 0 &| &1-a^2 \
      end{array} } right]
      $



      From $0=1-a^2$there we can obtain that the LES has a solution if $a=+/-1$



      Is this a valid solution to the problem and is there a free variable?







      matrices






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Jan 10 at 13:32









      SegonutSegonut

      62




      62






















          2 Answers
          2






          active

          oldest

          votes


















          0












          $begingroup$

          If $a=pm 1$, then indeed we have a solution.



          Now $a=1$, the second row become the zero row, and hence there is a free variable. Can you write down the general solution?



          If $a=-1$, the second row is not a zero row, there is no free variable. It has a unique solution.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
            $endgroup$
            – Segonut
            Jan 10 at 13:48










          • $begingroup$
            $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
            $endgroup$
            – Siong Thye Goh
            Jan 10 at 13:50










          • $begingroup$
            okay, thank you! And the transformations are correct?
            $endgroup$
            – Segonut
            Jan 10 at 13:50












          • $begingroup$
            And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
            $endgroup$
            – Segonut
            Jan 10 at 13:54










          • $begingroup$
            Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
            $endgroup$
            – Siong Thye Goh
            Jan 10 at 14:03



















          0












          $begingroup$

          begin{align}
          M=
          left[begin{array}{cc|c}
          a & a^2 & 1 \
          -1 & -1 & -a \
          1 & a & a
          end{array}right]
          &to
          left[begin{array}{cc|c}
          1 & a & a \
          -1 & -1 & -a \
          a & a^2 & 1
          end{array}right]
          &&R_1leftrightarrow R_3
          \[4px]&to
          left[begin{array}{cc|c}
          1 & a & a \
          0 & a-1 & 0 \
          0 & 0 & 1-a^2
          end{array}right]
          &&begin{aligned}R_2&gets R_2+R_1\R_3&gets R_3-aR_1end{aligned}
          end{align}

          Your work was pretty good.



          Now you have to distinguish the cases $a=1$, $a=-1$, $anepm1$



          Case $a=1$



          The matrix becomes
          $$
          left[begin{array}{cc|c}
          1 & 1 & 1 \
          0 & 0 & 0 \
          0 & 0 & 0
          end{array}right]
          $$

          and the system has infinitely many solutions
          $$
          begin{bmatrix} 1-h \ h end{bmatrix}=
          begin{bmatrix} 1 \ 0 end{bmatrix} + hbegin{bmatrix} -1 \ 1 end{bmatrix}
          $$



          Case $a=-1$



          The matrix becomes
          $$
          left[begin{array}{cc|c}
          1 & -1 & -1 \
          0 & 2 & 0 \
          0 & 0 & 0
          end{array}right]
          to
          left[begin{array}{cc|c}
          1 & 0 & -1 \
          0 & 1 & 0 \
          0 & 0 & 0
          end{array}right]
          $$

          and the system has a single solution $begin{bmatrix} -1 \ 0 end{bmatrix}$.



          Case $anepm1$



          No solution.






          share|cite|improve this answer









          $endgroup$













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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            0












            $begingroup$

            If $a=pm 1$, then indeed we have a solution.



            Now $a=1$, the second row become the zero row, and hence there is a free variable. Can you write down the general solution?



            If $a=-1$, the second row is not a zero row, there is no free variable. It has a unique solution.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
              $endgroup$
              – Segonut
              Jan 10 at 13:48










            • $begingroup$
              $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 13:50










            • $begingroup$
              okay, thank you! And the transformations are correct?
              $endgroup$
              – Segonut
              Jan 10 at 13:50












            • $begingroup$
              And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
              $endgroup$
              – Segonut
              Jan 10 at 13:54










            • $begingroup$
              Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 14:03
















            0












            $begingroup$

            If $a=pm 1$, then indeed we have a solution.



            Now $a=1$, the second row become the zero row, and hence there is a free variable. Can you write down the general solution?



            If $a=-1$, the second row is not a zero row, there is no free variable. It has a unique solution.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
              $endgroup$
              – Segonut
              Jan 10 at 13:48










            • $begingroup$
              $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 13:50










            • $begingroup$
              okay, thank you! And the transformations are correct?
              $endgroup$
              – Segonut
              Jan 10 at 13:50












            • $begingroup$
              And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
              $endgroup$
              – Segonut
              Jan 10 at 13:54










            • $begingroup$
              Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 14:03














            0












            0








            0





            $begingroup$

            If $a=pm 1$, then indeed we have a solution.



            Now $a=1$, the second row become the zero row, and hence there is a free variable. Can you write down the general solution?



            If $a=-1$, the second row is not a zero row, there is no free variable. It has a unique solution.






            share|cite|improve this answer









            $endgroup$



            If $a=pm 1$, then indeed we have a solution.



            Now $a=1$, the second row become the zero row, and hence there is a free variable. Can you write down the general solution?



            If $a=-1$, the second row is not a zero row, there is no free variable. It has a unique solution.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Jan 10 at 13:37









            Siong Thye GohSiong Thye Goh

            101k1466118




            101k1466118












            • $begingroup$
              but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
              $endgroup$
              – Segonut
              Jan 10 at 13:48










            • $begingroup$
              $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 13:50










            • $begingroup$
              okay, thank you! And the transformations are correct?
              $endgroup$
              – Segonut
              Jan 10 at 13:50












            • $begingroup$
              And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
              $endgroup$
              – Segonut
              Jan 10 at 13:54










            • $begingroup$
              Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 14:03


















            • $begingroup$
              but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
              $endgroup$
              – Segonut
              Jan 10 at 13:48










            • $begingroup$
              $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 13:50










            • $begingroup$
              okay, thank you! And the transformations are correct?
              $endgroup$
              – Segonut
              Jan 10 at 13:50












            • $begingroup$
              And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
              $endgroup$
              – Segonut
              Jan 10 at 13:54










            • $begingroup$
              Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
              $endgroup$
              – Siong Thye Goh
              Jan 10 at 14:03
















            $begingroup$
            but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
            $endgroup$
            – Segonut
            Jan 10 at 13:48




            $begingroup$
            but for $a=-1$ $x_2$ has to be $0$ so the row becomes a zero row again, doesn't it?
            $endgroup$
            – Segonut
            Jan 10 at 13:48












            $begingroup$
            $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
            $endgroup$
            – Siong Thye Goh
            Jan 10 at 13:50




            $begingroup$
            $x_2=0$ is true, but the second row is $[0, -2, 0]$ which is not a zero row.
            $endgroup$
            – Siong Thye Goh
            Jan 10 at 13:50












            $begingroup$
            okay, thank you! And the transformations are correct?
            $endgroup$
            – Segonut
            Jan 10 at 13:50






            $begingroup$
            okay, thank you! And the transformations are correct?
            $endgroup$
            – Segonut
            Jan 10 at 13:50














            $begingroup$
            And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
            $endgroup$
            – Segonut
            Jan 10 at 13:54




            $begingroup$
            And for $a=1$ the general solution should be $L=[1-t, t]$, shouldn't it? Because $x_1=1-x_2$ and $x_2=t$ (or any other name)? And for $a-1$ the solution is $[a,0]$, right?
            $endgroup$
            – Segonut
            Jan 10 at 13:54












            $begingroup$
            Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
            $endgroup$
            – Siong Thye Goh
            Jan 10 at 14:03




            $begingroup$
            Your Gaussian elimination step looks fine. if $a=1$, the general solution is indeed $(1-t,t)$ where $t$ is a free parameter. If $a=-1$, we have $x_2=0$, and the solution is $(a,0)=(-1,0)$.
            $endgroup$
            – Siong Thye Goh
            Jan 10 at 14:03











            0












            $begingroup$

            begin{align}
            M=
            left[begin{array}{cc|c}
            a & a^2 & 1 \
            -1 & -1 & -a \
            1 & a & a
            end{array}right]
            &to
            left[begin{array}{cc|c}
            1 & a & a \
            -1 & -1 & -a \
            a & a^2 & 1
            end{array}right]
            &&R_1leftrightarrow R_3
            \[4px]&to
            left[begin{array}{cc|c}
            1 & a & a \
            0 & a-1 & 0 \
            0 & 0 & 1-a^2
            end{array}right]
            &&begin{aligned}R_2&gets R_2+R_1\R_3&gets R_3-aR_1end{aligned}
            end{align}

            Your work was pretty good.



            Now you have to distinguish the cases $a=1$, $a=-1$, $anepm1$



            Case $a=1$



            The matrix becomes
            $$
            left[begin{array}{cc|c}
            1 & 1 & 1 \
            0 & 0 & 0 \
            0 & 0 & 0
            end{array}right]
            $$

            and the system has infinitely many solutions
            $$
            begin{bmatrix} 1-h \ h end{bmatrix}=
            begin{bmatrix} 1 \ 0 end{bmatrix} + hbegin{bmatrix} -1 \ 1 end{bmatrix}
            $$



            Case $a=-1$



            The matrix becomes
            $$
            left[begin{array}{cc|c}
            1 & -1 & -1 \
            0 & 2 & 0 \
            0 & 0 & 0
            end{array}right]
            to
            left[begin{array}{cc|c}
            1 & 0 & -1 \
            0 & 1 & 0 \
            0 & 0 & 0
            end{array}right]
            $$

            and the system has a single solution $begin{bmatrix} -1 \ 0 end{bmatrix}$.



            Case $anepm1$



            No solution.






            share|cite|improve this answer









            $endgroup$


















              0












              $begingroup$

              begin{align}
              M=
              left[begin{array}{cc|c}
              a & a^2 & 1 \
              -1 & -1 & -a \
              1 & a & a
              end{array}right]
              &to
              left[begin{array}{cc|c}
              1 & a & a \
              -1 & -1 & -a \
              a & a^2 & 1
              end{array}right]
              &&R_1leftrightarrow R_3
              \[4px]&to
              left[begin{array}{cc|c}
              1 & a & a \
              0 & a-1 & 0 \
              0 & 0 & 1-a^2
              end{array}right]
              &&begin{aligned}R_2&gets R_2+R_1\R_3&gets R_3-aR_1end{aligned}
              end{align}

              Your work was pretty good.



              Now you have to distinguish the cases $a=1$, $a=-1$, $anepm1$



              Case $a=1$



              The matrix becomes
              $$
              left[begin{array}{cc|c}
              1 & 1 & 1 \
              0 & 0 & 0 \
              0 & 0 & 0
              end{array}right]
              $$

              and the system has infinitely many solutions
              $$
              begin{bmatrix} 1-h \ h end{bmatrix}=
              begin{bmatrix} 1 \ 0 end{bmatrix} + hbegin{bmatrix} -1 \ 1 end{bmatrix}
              $$



              Case $a=-1$



              The matrix becomes
              $$
              left[begin{array}{cc|c}
              1 & -1 & -1 \
              0 & 2 & 0 \
              0 & 0 & 0
              end{array}right]
              to
              left[begin{array}{cc|c}
              1 & 0 & -1 \
              0 & 1 & 0 \
              0 & 0 & 0
              end{array}right]
              $$

              and the system has a single solution $begin{bmatrix} -1 \ 0 end{bmatrix}$.



              Case $anepm1$



              No solution.






              share|cite|improve this answer









              $endgroup$
















                0












                0








                0





                $begingroup$

                begin{align}
                M=
                left[begin{array}{cc|c}
                a & a^2 & 1 \
                -1 & -1 & -a \
                1 & a & a
                end{array}right]
                &to
                left[begin{array}{cc|c}
                1 & a & a \
                -1 & -1 & -a \
                a & a^2 & 1
                end{array}right]
                &&R_1leftrightarrow R_3
                \[4px]&to
                left[begin{array}{cc|c}
                1 & a & a \
                0 & a-1 & 0 \
                0 & 0 & 1-a^2
                end{array}right]
                &&begin{aligned}R_2&gets R_2+R_1\R_3&gets R_3-aR_1end{aligned}
                end{align}

                Your work was pretty good.



                Now you have to distinguish the cases $a=1$, $a=-1$, $anepm1$



                Case $a=1$



                The matrix becomes
                $$
                left[begin{array}{cc|c}
                1 & 1 & 1 \
                0 & 0 & 0 \
                0 & 0 & 0
                end{array}right]
                $$

                and the system has infinitely many solutions
                $$
                begin{bmatrix} 1-h \ h end{bmatrix}=
                begin{bmatrix} 1 \ 0 end{bmatrix} + hbegin{bmatrix} -1 \ 1 end{bmatrix}
                $$



                Case $a=-1$



                The matrix becomes
                $$
                left[begin{array}{cc|c}
                1 & -1 & -1 \
                0 & 2 & 0 \
                0 & 0 & 0
                end{array}right]
                to
                left[begin{array}{cc|c}
                1 & 0 & -1 \
                0 & 1 & 0 \
                0 & 0 & 0
                end{array}right]
                $$

                and the system has a single solution $begin{bmatrix} -1 \ 0 end{bmatrix}$.



                Case $anepm1$



                No solution.






                share|cite|improve this answer









                $endgroup$



                begin{align}
                M=
                left[begin{array}{cc|c}
                a & a^2 & 1 \
                -1 & -1 & -a \
                1 & a & a
                end{array}right]
                &to
                left[begin{array}{cc|c}
                1 & a & a \
                -1 & -1 & -a \
                a & a^2 & 1
                end{array}right]
                &&R_1leftrightarrow R_3
                \[4px]&to
                left[begin{array}{cc|c}
                1 & a & a \
                0 & a-1 & 0 \
                0 & 0 & 1-a^2
                end{array}right]
                &&begin{aligned}R_2&gets R_2+R_1\R_3&gets R_3-aR_1end{aligned}
                end{align}

                Your work was pretty good.



                Now you have to distinguish the cases $a=1$, $a=-1$, $anepm1$



                Case $a=1$



                The matrix becomes
                $$
                left[begin{array}{cc|c}
                1 & 1 & 1 \
                0 & 0 & 0 \
                0 & 0 & 0
                end{array}right]
                $$

                and the system has infinitely many solutions
                $$
                begin{bmatrix} 1-h \ h end{bmatrix}=
                begin{bmatrix} 1 \ 0 end{bmatrix} + hbegin{bmatrix} -1 \ 1 end{bmatrix}
                $$



                Case $a=-1$



                The matrix becomes
                $$
                left[begin{array}{cc|c}
                1 & -1 & -1 \
                0 & 2 & 0 \
                0 & 0 & 0
                end{array}right]
                to
                left[begin{array}{cc|c}
                1 & 0 & -1 \
                0 & 1 & 0 \
                0 & 0 & 0
                end{array}right]
                $$

                and the system has a single solution $begin{bmatrix} -1 \ 0 end{bmatrix}$.



                Case $anepm1$



                No solution.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 10 at 14:58









                egregegreg

                181k1485203




                181k1485203






























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