Orthogonal projection and orthogonal complements onto a plane

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Let $V$ be a subspace of $mathbbR^n$ with $V ne mathbbR^n$ and $Vne 0$. Let $A$ be the matrix of the linear transformation $textproj_V : mathbbR^ntomathbbR^n$ that is the projection onto $V$.



Calculate $A$ in the case where $n=3$ and $V$ is given by the equation $x+y+z=0$.



I am not really sure how to approach this question. I am aware that the normal vector will be $(1,1,1)$. And I know that the orthogonal projection onto the plane $V$ given by the equation $x+y+z=0$ is equal to the identity minus the orthogonal projection onto the orthogonal complement. I believe the orthogonal complement will be in the span of the normal vector (not entirley sure why?) but I am also unsure how to find the orthogonal complement.







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    Let $V$ be a subspace of $mathbbR^n$ with $V ne mathbbR^n$ and $Vne 0$. Let $A$ be the matrix of the linear transformation $textproj_V : mathbbR^ntomathbbR^n$ that is the projection onto $V$.



    Calculate $A$ in the case where $n=3$ and $V$ is given by the equation $x+y+z=0$.



    I am not really sure how to approach this question. I am aware that the normal vector will be $(1,1,1)$. And I know that the orthogonal projection onto the plane $V$ given by the equation $x+y+z=0$ is equal to the identity minus the orthogonal projection onto the orthogonal complement. I believe the orthogonal complement will be in the span of the normal vector (not entirley sure why?) but I am also unsure how to find the orthogonal complement.







    share|cite|improve this question























      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      Let $V$ be a subspace of $mathbbR^n$ with $V ne mathbbR^n$ and $Vne 0$. Let $A$ be the matrix of the linear transformation $textproj_V : mathbbR^ntomathbbR^n$ that is the projection onto $V$.



      Calculate $A$ in the case where $n=3$ and $V$ is given by the equation $x+y+z=0$.



      I am not really sure how to approach this question. I am aware that the normal vector will be $(1,1,1)$. And I know that the orthogonal projection onto the plane $V$ given by the equation $x+y+z=0$ is equal to the identity minus the orthogonal projection onto the orthogonal complement. I believe the orthogonal complement will be in the span of the normal vector (not entirley sure why?) but I am also unsure how to find the orthogonal complement.







      share|cite|improve this question













      Let $V$ be a subspace of $mathbbR^n$ with $V ne mathbbR^n$ and $Vne 0$. Let $A$ be the matrix of the linear transformation $textproj_V : mathbbR^ntomathbbR^n$ that is the projection onto $V$.



      Calculate $A$ in the case where $n=3$ and $V$ is given by the equation $x+y+z=0$.



      I am not really sure how to approach this question. I am aware that the normal vector will be $(1,1,1)$. And I know that the orthogonal projection onto the plane $V$ given by the equation $x+y+z=0$ is equal to the identity minus the orthogonal projection onto the orthogonal complement. I believe the orthogonal complement will be in the span of the normal vector (not entirley sure why?) but I am also unsure how to find the orthogonal complement.









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      share|cite|improve this question




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      edited Jul 27 at 13:10









      mechanodroid

      22.2k52041




      22.2k52041









      asked Jul 27 at 12:28









      Molly

      303




      303




















          5 Answers
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          Hint:



          The projection of a vector $vec u$ onto the normal to the plane $x+y+z=0$, with normal vector $vec n=(1,1,1)$, is given by the formula:
          $$p_vec n(vec u)=vec u-fracvec ncdotvec uvec ncdotvec n,vec n.$$
          Apply this formula to each vector of the canonical basis to find the column vectors of the projection matrix.






          share|cite|improve this answer




























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            You are right: $(1,1,1)$ is orthogonal to $V$. Therefore, $A.(1,1,1)=(0,0,0)$. Now, consider the vectors $(1,-1,0)$ and $(1,0,-1)$. Since they both belong to $V$, you must have $A.(1,-1,0)=(1,-1,0)$ and $A.(1,0,-1)=(1,0,-1)$.



            Now, since$$(1,0,0)=frac13(1,1,1)+frac13(1,-1,0)+frac13(1,0,-1),$$you must have$$A.(1,0,0)=frac13(1,-1,0)+frac13(1,0,-1)=left(frac23,-frac13,-frac13right).$$So, the entries of the first column of the matrix of $operatornameproj_V$ with respect to the standard basis will be $frac23$, $-frac13$ and $-frac13$. Can you take it from here?






            share|cite|improve this answer





















            • Where are you getting the 1/3 from?
              – Molly
              Jul 27 at 20:50










            • From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
              – José Carlos Santos
              Jul 27 at 21:11

















            up vote
            0
            down vote













            As you say, $u:=(1,1,1)^T$ is a normal vector to the plane, thus it will span the orthogonal complement.



            Its projection is given by $varphi:vmapstofracu^Tvu^Tuu$.

            You can check that it fixes $u$, but gives $0$ whenever $vperp u$.

            To obtain its matrix in the standard basis, just calculate $varphi(e_i)$ for the standard basis $e_1,e_2,e_3$.






            share|cite|improve this answer




























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              $leftfrac1sqrt2(1,-1,0), frac1sqrt6(1,1,-2)right$ is an orthonormal basis for $V$ so the projection is given by



              beginalign
              P(x,y,z) &= leftlangle (x,y,z), frac1sqrt2(1,-1,0)rightranglefrac1sqrt2(1,-1,0)+leftlangle (x,y,z), frac1sqrt6(1,1,-2)rightranglefrac1sqrt6(1,1,-2) \
              &= frac12 (x-y, -x+y,0) + frac16(x+y-2z,x+y-2z,-x-y+4z)\
              &= frac16 (4x-2y-2z, -2x+4y-2z, -2x-2y+4z)
              endalign



              so $$A = frac16beginbmatrix 4 & -2 & -2 \ -2 & 4 & -2 \ -2 & -2 & 4 endbmatrix$$






              share|cite|improve this answer






























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                The orthogonal projection of $(x,y,z)$ onto the plane through the origin with normal $(1,1,1)$ is the unique $(x',y',z')$ on the plane such that
                $$
                ((x,y,z)-(x',y',z')) = c(1,1,1), \
                x'+y'+z'=0.
                $$
                Dotting the first equation with $(1,1,1)$ and using $x'+y'+z'=0$ gives
                $$
                c = frac13(x+y+z).
                $$



                So the projection of $(x,y,z)$ onto the given plane is



                beginalign
                (x',y',z') &= (x,y,z)-frac13(x+y+z)(1,1,1) \
                &= frac13(2x-y-z,-x+2y-z,-x-y+2z).
                endalign






                share|cite|improve this answer





















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                  5 Answers
                  5






                  active

                  oldest

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                  5 Answers
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                  active

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                  active

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                  up vote
                  1
                  down vote













                  Hint:



                  The projection of a vector $vec u$ onto the normal to the plane $x+y+z=0$, with normal vector $vec n=(1,1,1)$, is given by the formula:
                  $$p_vec n(vec u)=vec u-fracvec ncdotvec uvec ncdotvec n,vec n.$$
                  Apply this formula to each vector of the canonical basis to find the column vectors of the projection matrix.






                  share|cite|improve this answer

























                    up vote
                    1
                    down vote













                    Hint:



                    The projection of a vector $vec u$ onto the normal to the plane $x+y+z=0$, with normal vector $vec n=(1,1,1)$, is given by the formula:
                    $$p_vec n(vec u)=vec u-fracvec ncdotvec uvec ncdotvec n,vec n.$$
                    Apply this formula to each vector of the canonical basis to find the column vectors of the projection matrix.






                    share|cite|improve this answer























                      up vote
                      1
                      down vote










                      up vote
                      1
                      down vote









                      Hint:



                      The projection of a vector $vec u$ onto the normal to the plane $x+y+z=0$, with normal vector $vec n=(1,1,1)$, is given by the formula:
                      $$p_vec n(vec u)=vec u-fracvec ncdotvec uvec ncdotvec n,vec n.$$
                      Apply this formula to each vector of the canonical basis to find the column vectors of the projection matrix.






                      share|cite|improve this answer













                      Hint:



                      The projection of a vector $vec u$ onto the normal to the plane $x+y+z=0$, with normal vector $vec n=(1,1,1)$, is given by the formula:
                      $$p_vec n(vec u)=vec u-fracvec ncdotvec uvec ncdotvec n,vec n.$$
                      Apply this formula to each vector of the canonical basis to find the column vectors of the projection matrix.







                      share|cite|improve this answer













                      share|cite|improve this answer



                      share|cite|improve this answer











                      answered Jul 27 at 12:42









                      Bernard

                      110k635102




                      110k635102




















                          up vote
                          0
                          down vote













                          You are right: $(1,1,1)$ is orthogonal to $V$. Therefore, $A.(1,1,1)=(0,0,0)$. Now, consider the vectors $(1,-1,0)$ and $(1,0,-1)$. Since they both belong to $V$, you must have $A.(1,-1,0)=(1,-1,0)$ and $A.(1,0,-1)=(1,0,-1)$.



                          Now, since$$(1,0,0)=frac13(1,1,1)+frac13(1,-1,0)+frac13(1,0,-1),$$you must have$$A.(1,0,0)=frac13(1,-1,0)+frac13(1,0,-1)=left(frac23,-frac13,-frac13right).$$So, the entries of the first column of the matrix of $operatornameproj_V$ with respect to the standard basis will be $frac23$, $-frac13$ and $-frac13$. Can you take it from here?






                          share|cite|improve this answer





















                          • Where are you getting the 1/3 from?
                            – Molly
                            Jul 27 at 20:50










                          • From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
                            – José Carlos Santos
                            Jul 27 at 21:11














                          up vote
                          0
                          down vote













                          You are right: $(1,1,1)$ is orthogonal to $V$. Therefore, $A.(1,1,1)=(0,0,0)$. Now, consider the vectors $(1,-1,0)$ and $(1,0,-1)$. Since they both belong to $V$, you must have $A.(1,-1,0)=(1,-1,0)$ and $A.(1,0,-1)=(1,0,-1)$.



                          Now, since$$(1,0,0)=frac13(1,1,1)+frac13(1,-1,0)+frac13(1,0,-1),$$you must have$$A.(1,0,0)=frac13(1,-1,0)+frac13(1,0,-1)=left(frac23,-frac13,-frac13right).$$So, the entries of the first column of the matrix of $operatornameproj_V$ with respect to the standard basis will be $frac23$, $-frac13$ and $-frac13$. Can you take it from here?






                          share|cite|improve this answer





















                          • Where are you getting the 1/3 from?
                            – Molly
                            Jul 27 at 20:50










                          • From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
                            – José Carlos Santos
                            Jul 27 at 21:11












                          up vote
                          0
                          down vote










                          up vote
                          0
                          down vote









                          You are right: $(1,1,1)$ is orthogonal to $V$. Therefore, $A.(1,1,1)=(0,0,0)$. Now, consider the vectors $(1,-1,0)$ and $(1,0,-1)$. Since they both belong to $V$, you must have $A.(1,-1,0)=(1,-1,0)$ and $A.(1,0,-1)=(1,0,-1)$.



                          Now, since$$(1,0,0)=frac13(1,1,1)+frac13(1,-1,0)+frac13(1,0,-1),$$you must have$$A.(1,0,0)=frac13(1,-1,0)+frac13(1,0,-1)=left(frac23,-frac13,-frac13right).$$So, the entries of the first column of the matrix of $operatornameproj_V$ with respect to the standard basis will be $frac23$, $-frac13$ and $-frac13$. Can you take it from here?






                          share|cite|improve this answer













                          You are right: $(1,1,1)$ is orthogonal to $V$. Therefore, $A.(1,1,1)=(0,0,0)$. Now, consider the vectors $(1,-1,0)$ and $(1,0,-1)$. Since they both belong to $V$, you must have $A.(1,-1,0)=(1,-1,0)$ and $A.(1,0,-1)=(1,0,-1)$.



                          Now, since$$(1,0,0)=frac13(1,1,1)+frac13(1,-1,0)+frac13(1,0,-1),$$you must have$$A.(1,0,0)=frac13(1,-1,0)+frac13(1,0,-1)=left(frac23,-frac13,-frac13right).$$So, the entries of the first column of the matrix of $operatornameproj_V$ with respect to the standard basis will be $frac23$, $-frac13$ and $-frac13$. Can you take it from here?







                          share|cite|improve this answer













                          share|cite|improve this answer



                          share|cite|improve this answer











                          answered Jul 27 at 12:37









                          José Carlos Santos

                          113k1696173




                          113k1696173











                          • Where are you getting the 1/3 from?
                            – Molly
                            Jul 27 at 20:50










                          • From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
                            – José Carlos Santos
                            Jul 27 at 21:11
















                          • Where are you getting the 1/3 from?
                            – Molly
                            Jul 27 at 20:50










                          • From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
                            – José Carlos Santos
                            Jul 27 at 21:11















                          Where are you getting the 1/3 from?
                          – Molly
                          Jul 27 at 20:50




                          Where are you getting the 1/3 from?
                          – Molly
                          Jul 27 at 20:50












                          From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
                          – José Carlos Santos
                          Jul 27 at 21:11




                          From solving the system $(1,0,0)=alpha(1,1,1)+beta(1,-1,0)+gamma(1,0,-1)$.
                          – José Carlos Santos
                          Jul 27 at 21:11










                          up vote
                          0
                          down vote













                          As you say, $u:=(1,1,1)^T$ is a normal vector to the plane, thus it will span the orthogonal complement.



                          Its projection is given by $varphi:vmapstofracu^Tvu^Tuu$.

                          You can check that it fixes $u$, but gives $0$ whenever $vperp u$.

                          To obtain its matrix in the standard basis, just calculate $varphi(e_i)$ for the standard basis $e_1,e_2,e_3$.






                          share|cite|improve this answer

























                            up vote
                            0
                            down vote













                            As you say, $u:=(1,1,1)^T$ is a normal vector to the plane, thus it will span the orthogonal complement.



                            Its projection is given by $varphi:vmapstofracu^Tvu^Tuu$.

                            You can check that it fixes $u$, but gives $0$ whenever $vperp u$.

                            To obtain its matrix in the standard basis, just calculate $varphi(e_i)$ for the standard basis $e_1,e_2,e_3$.






                            share|cite|improve this answer























                              up vote
                              0
                              down vote










                              up vote
                              0
                              down vote









                              As you say, $u:=(1,1,1)^T$ is a normal vector to the plane, thus it will span the orthogonal complement.



                              Its projection is given by $varphi:vmapstofracu^Tvu^Tuu$.

                              You can check that it fixes $u$, but gives $0$ whenever $vperp u$.

                              To obtain its matrix in the standard basis, just calculate $varphi(e_i)$ for the standard basis $e_1,e_2,e_3$.






                              share|cite|improve this answer













                              As you say, $u:=(1,1,1)^T$ is a normal vector to the plane, thus it will span the orthogonal complement.



                              Its projection is given by $varphi:vmapstofracu^Tvu^Tuu$.

                              You can check that it fixes $u$, but gives $0$ whenever $vperp u$.

                              To obtain its matrix in the standard basis, just calculate $varphi(e_i)$ for the standard basis $e_1,e_2,e_3$.







                              share|cite|improve this answer













                              share|cite|improve this answer



                              share|cite|improve this answer











                              answered Jul 27 at 12:41









                              Berci

                              56.4k23570




                              56.4k23570




















                                  up vote
                                  0
                                  down vote













                                  $leftfrac1sqrt2(1,-1,0), frac1sqrt6(1,1,-2)right$ is an orthonormal basis for $V$ so the projection is given by



                                  beginalign
                                  P(x,y,z) &= leftlangle (x,y,z), frac1sqrt2(1,-1,0)rightranglefrac1sqrt2(1,-1,0)+leftlangle (x,y,z), frac1sqrt6(1,1,-2)rightranglefrac1sqrt6(1,1,-2) \
                                  &= frac12 (x-y, -x+y,0) + frac16(x+y-2z,x+y-2z,-x-y+4z)\
                                  &= frac16 (4x-2y-2z, -2x+4y-2z, -2x-2y+4z)
                                  endalign



                                  so $$A = frac16beginbmatrix 4 & -2 & -2 \ -2 & 4 & -2 \ -2 & -2 & 4 endbmatrix$$






                                  share|cite|improve this answer



























                                    up vote
                                    0
                                    down vote













                                    $leftfrac1sqrt2(1,-1,0), frac1sqrt6(1,1,-2)right$ is an orthonormal basis for $V$ so the projection is given by



                                    beginalign
                                    P(x,y,z) &= leftlangle (x,y,z), frac1sqrt2(1,-1,0)rightranglefrac1sqrt2(1,-1,0)+leftlangle (x,y,z), frac1sqrt6(1,1,-2)rightranglefrac1sqrt6(1,1,-2) \
                                    &= frac12 (x-y, -x+y,0) + frac16(x+y-2z,x+y-2z,-x-y+4z)\
                                    &= frac16 (4x-2y-2z, -2x+4y-2z, -2x-2y+4z)
                                    endalign



                                    so $$A = frac16beginbmatrix 4 & -2 & -2 \ -2 & 4 & -2 \ -2 & -2 & 4 endbmatrix$$






                                    share|cite|improve this answer

























                                      up vote
                                      0
                                      down vote










                                      up vote
                                      0
                                      down vote









                                      $leftfrac1sqrt2(1,-1,0), frac1sqrt6(1,1,-2)right$ is an orthonormal basis for $V$ so the projection is given by



                                      beginalign
                                      P(x,y,z) &= leftlangle (x,y,z), frac1sqrt2(1,-1,0)rightranglefrac1sqrt2(1,-1,0)+leftlangle (x,y,z), frac1sqrt6(1,1,-2)rightranglefrac1sqrt6(1,1,-2) \
                                      &= frac12 (x-y, -x+y,0) + frac16(x+y-2z,x+y-2z,-x-y+4z)\
                                      &= frac16 (4x-2y-2z, -2x+4y-2z, -2x-2y+4z)
                                      endalign



                                      so $$A = frac16beginbmatrix 4 & -2 & -2 \ -2 & 4 & -2 \ -2 & -2 & 4 endbmatrix$$






                                      share|cite|improve this answer















                                      $leftfrac1sqrt2(1,-1,0), frac1sqrt6(1,1,-2)right$ is an orthonormal basis for $V$ so the projection is given by



                                      beginalign
                                      P(x,y,z) &= leftlangle (x,y,z), frac1sqrt2(1,-1,0)rightranglefrac1sqrt2(1,-1,0)+leftlangle (x,y,z), frac1sqrt6(1,1,-2)rightranglefrac1sqrt6(1,1,-2) \
                                      &= frac12 (x-y, -x+y,0) + frac16(x+y-2z,x+y-2z,-x-y+4z)\
                                      &= frac16 (4x-2y-2z, -2x+4y-2z, -2x-2y+4z)
                                      endalign



                                      so $$A = frac16beginbmatrix 4 & -2 & -2 \ -2 & 4 & -2 \ -2 & -2 & 4 endbmatrix$$







                                      share|cite|improve this answer















                                      share|cite|improve this answer



                                      share|cite|improve this answer








                                      edited Jul 27 at 13:09


























                                      answered Jul 27 at 12:49









                                      mechanodroid

                                      22.2k52041




                                      22.2k52041




















                                          up vote
                                          0
                                          down vote













                                          The orthogonal projection of $(x,y,z)$ onto the plane through the origin with normal $(1,1,1)$ is the unique $(x',y',z')$ on the plane such that
                                          $$
                                          ((x,y,z)-(x',y',z')) = c(1,1,1), \
                                          x'+y'+z'=0.
                                          $$
                                          Dotting the first equation with $(1,1,1)$ and using $x'+y'+z'=0$ gives
                                          $$
                                          c = frac13(x+y+z).
                                          $$



                                          So the projection of $(x,y,z)$ onto the given plane is



                                          beginalign
                                          (x',y',z') &= (x,y,z)-frac13(x+y+z)(1,1,1) \
                                          &= frac13(2x-y-z,-x+2y-z,-x-y+2z).
                                          endalign






                                          share|cite|improve this answer

























                                            up vote
                                            0
                                            down vote













                                            The orthogonal projection of $(x,y,z)$ onto the plane through the origin with normal $(1,1,1)$ is the unique $(x',y',z')$ on the plane such that
                                            $$
                                            ((x,y,z)-(x',y',z')) = c(1,1,1), \
                                            x'+y'+z'=0.
                                            $$
                                            Dotting the first equation with $(1,1,1)$ and using $x'+y'+z'=0$ gives
                                            $$
                                            c = frac13(x+y+z).
                                            $$



                                            So the projection of $(x,y,z)$ onto the given plane is



                                            beginalign
                                            (x',y',z') &= (x,y,z)-frac13(x+y+z)(1,1,1) \
                                            &= frac13(2x-y-z,-x+2y-z,-x-y+2z).
                                            endalign






                                            share|cite|improve this answer























                                              up vote
                                              0
                                              down vote










                                              up vote
                                              0
                                              down vote









                                              The orthogonal projection of $(x,y,z)$ onto the plane through the origin with normal $(1,1,1)$ is the unique $(x',y',z')$ on the plane such that
                                              $$
                                              ((x,y,z)-(x',y',z')) = c(1,1,1), \
                                              x'+y'+z'=0.
                                              $$
                                              Dotting the first equation with $(1,1,1)$ and using $x'+y'+z'=0$ gives
                                              $$
                                              c = frac13(x+y+z).
                                              $$



                                              So the projection of $(x,y,z)$ onto the given plane is



                                              beginalign
                                              (x',y',z') &= (x,y,z)-frac13(x+y+z)(1,1,1) \
                                              &= frac13(2x-y-z,-x+2y-z,-x-y+2z).
                                              endalign






                                              share|cite|improve this answer













                                              The orthogonal projection of $(x,y,z)$ onto the plane through the origin with normal $(1,1,1)$ is the unique $(x',y',z')$ on the plane such that
                                              $$
                                              ((x,y,z)-(x',y',z')) = c(1,1,1), \
                                              x'+y'+z'=0.
                                              $$
                                              Dotting the first equation with $(1,1,1)$ and using $x'+y'+z'=0$ gives
                                              $$
                                              c = frac13(x+y+z).
                                              $$



                                              So the projection of $(x,y,z)$ onto the given plane is



                                              beginalign
                                              (x',y',z') &= (x,y,z)-frac13(x+y+z)(1,1,1) \
                                              &= frac13(2x-y-z,-x+2y-z,-x-y+2z).
                                              endalign







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                                              answered Jul 28 at 5:11









                                              DisintegratingByParts

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