Find the maximum value of the function $M=sqrt{x^2+y^2-2xy}+sqrt{y^2+z^2-2yz}+sqrt{z^2+x^2-2xz}$












0












$begingroup$


Let $0le x;y;zle 3$. Find maximum value of function $$M=sqrt{x^2+y^2-2xy}+sqrt{y^2+z^2-2yz}+sqrt{z^2+x^2-2xz}$$





I can rewrite the function $M=|x-y|+|y-z|+|z-x|$



WLOG $0le xle yle zle 3$ so we have $M=y-x+z-y+z-x=2z-2x$



I dont know how to use $0le x;y;zle 3$ to evaluate the last inequality.



My idea : $M=|x-y|+|y-z|+|z-x|le sqrt{((x-y)^2+(y-z)^2+(x-z)^2)(1+1+1)}$



"=" occurs when $|x-y|=|y-z|=|z-x|$ but wrong.










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$endgroup$












  • $begingroup$
    M = 2(z-x) then z=3 and x=0
    $endgroup$
    – eyllanesc
    Dec 22 '18 at 4:28


















0












$begingroup$


Let $0le x;y;zle 3$. Find maximum value of function $$M=sqrt{x^2+y^2-2xy}+sqrt{y^2+z^2-2yz}+sqrt{z^2+x^2-2xz}$$





I can rewrite the function $M=|x-y|+|y-z|+|z-x|$



WLOG $0le xle yle zle 3$ so we have $M=y-x+z-y+z-x=2z-2x$



I dont know how to use $0le x;y;zle 3$ to evaluate the last inequality.



My idea : $M=|x-y|+|y-z|+|z-x|le sqrt{((x-y)^2+(y-z)^2+(x-z)^2)(1+1+1)}$



"=" occurs when $|x-y|=|y-z|=|z-x|$ but wrong.










share|cite|improve this question











$endgroup$












  • $begingroup$
    M = 2(z-x) then z=3 and x=0
    $endgroup$
    – eyllanesc
    Dec 22 '18 at 4:28
















0












0








0


1



$begingroup$


Let $0le x;y;zle 3$. Find maximum value of function $$M=sqrt{x^2+y^2-2xy}+sqrt{y^2+z^2-2yz}+sqrt{z^2+x^2-2xz}$$





I can rewrite the function $M=|x-y|+|y-z|+|z-x|$



WLOG $0le xle yle zle 3$ so we have $M=y-x+z-y+z-x=2z-2x$



I dont know how to use $0le x;y;zle 3$ to evaluate the last inequality.



My idea : $M=|x-y|+|y-z|+|z-x|le sqrt{((x-y)^2+(y-z)^2+(x-z)^2)(1+1+1)}$



"=" occurs when $|x-y|=|y-z|=|z-x|$ but wrong.










share|cite|improve this question











$endgroup$




Let $0le x;y;zle 3$. Find maximum value of function $$M=sqrt{x^2+y^2-2xy}+sqrt{y^2+z^2-2yz}+sqrt{z^2+x^2-2xz}$$





I can rewrite the function $M=|x-y|+|y-z|+|z-x|$



WLOG $0le xle yle zle 3$ so we have $M=y-x+z-y+z-x=2z-2x$



I dont know how to use $0le x;y;zle 3$ to evaluate the last inequality.



My idea : $M=|x-y|+|y-z|+|z-x|le sqrt{((x-y)^2+(y-z)^2+(x-z)^2)(1+1+1)}$



"=" occurs when $|x-y|=|y-z|=|z-x|$ but wrong.







inequality radicals maxima-minima






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edited Dec 24 '18 at 17:11









Michael Rozenberg

107k1894198




107k1894198










asked Dec 22 '18 at 3:50









nDLynknDLynk

20010




20010












  • $begingroup$
    M = 2(z-x) then z=3 and x=0
    $endgroup$
    – eyllanesc
    Dec 22 '18 at 4:28




















  • $begingroup$
    M = 2(z-x) then z=3 and x=0
    $endgroup$
    – eyllanesc
    Dec 22 '18 at 4:28


















$begingroup$
M = 2(z-x) then z=3 and x=0
$endgroup$
– eyllanesc
Dec 22 '18 at 4:28






$begingroup$
M = 2(z-x) then z=3 and x=0
$endgroup$
– eyllanesc
Dec 22 '18 at 4:28












4 Answers
4






active

oldest

votes


















3












$begingroup$

You need to make also one step only:



Let $y=y=0$ and $z=3$.



Thus, we got a value $6$.



We'll prove that it'a maximal value.



Indeed, let $xgeq ygeq z$.



Thus, we need to prove that
$$x-y+x-z+y-zleq6$$ or $$x-zleq3,$$ which is true because
$$x-zleq xleq3.$$






share|cite|improve this answer









$endgroup$





















    2












    $begingroup$

    Note that
    $$
    left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = M = const
    $$

    is a cylinder with axis $x=y=z$.



    In fact
    $$
    eqalign{
    & M = const = left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = cr
    & = left| {left( {x - a} right) - left( {y - a} right)} right| + left| {left( {y - a} right) - left( {z - a} right)} right|
    + left| {left( {z - a} right) - left( {x - a} right)} right| cr}
    $$



    Sq_Cylinder_1



    The scheme tells you how you can determine the maximum within the given bounds.

    For instance we see that, for $y=0$ we get
    $$
    eqalign{
    & left{ matrix{
    y = 0 hfill cr
    0 le z,x hfill cr} right.quad Rightarrow quad left| x right| + left| z right| + left| {z - x} right| = 2left| z right| = Mquad Rightarrow cr
    & Rightarrow quad max M = 6quad left| matrix{
    ;0 le forall x le 3 hfill cr
    ;y = 0 hfill cr
    ;z = 3 hfill cr} right. cr}
    $$

    and the other sets of solutions will be symmetrical to this.






    share|cite|improve this answer









    $endgroup$





















      1












      $begingroup$

      You have $M=2z-2x $ as you have done
      so we need to have $x=0$ in the maximal value of $M$
      so $$Mle 2z$$
      and $$zle 3$$
      hence $$Mle 6$$






      share|cite|improve this answer









      $endgroup$





















        1












        $begingroup$

        This solution might be a bit far reaching but the idea behind it may also help you in proving many other inequalities.



        The following facts give almost immediately the answer to your problem:





        • $M(x,y,z) = |x-y|+|y-z|+|z-x|$ is a convex function.


        • $Q = {(x,y,z) in mathbb{R}^3; | ; 0leq x,y,z leq 3 }$ is a compact (bounded and closed) convex set.

        • As $M$ is a continuous function on the compact set $Q$, $M$ has a maximum on $Q$.

        • Since $M$ and $Q$ are convex, the maximum is attained in an extremal point of $Q$ (the "corners" of the cube $Q$).

        • The corners of the cube $Q$ are $C = {(c_x,c_y,c_z) in mathbb{R}^3; | ; c_x,c_y,c_z in {0,3} } $


        By checking the values of $M$ at the corners you find the maximum at the corners $(3,0,0), (0,3,0), (0,0,3)$:
        $$max_Q M = max_C M = 6$$






        share|cite|improve this answer









        $endgroup$













          Your Answer





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          4 Answers
          4






          active

          oldest

          votes








          4 Answers
          4






          active

          oldest

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          active

          oldest

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          active

          oldest

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          3












          $begingroup$

          You need to make also one step only:



          Let $y=y=0$ and $z=3$.



          Thus, we got a value $6$.



          We'll prove that it'a maximal value.



          Indeed, let $xgeq ygeq z$.



          Thus, we need to prove that
          $$x-y+x-z+y-zleq6$$ or $$x-zleq3,$$ which is true because
          $$x-zleq xleq3.$$






          share|cite|improve this answer









          $endgroup$


















            3












            $begingroup$

            You need to make also one step only:



            Let $y=y=0$ and $z=3$.



            Thus, we got a value $6$.



            We'll prove that it'a maximal value.



            Indeed, let $xgeq ygeq z$.



            Thus, we need to prove that
            $$x-y+x-z+y-zleq6$$ or $$x-zleq3,$$ which is true because
            $$x-zleq xleq3.$$






            share|cite|improve this answer









            $endgroup$
















              3












              3








              3





              $begingroup$

              You need to make also one step only:



              Let $y=y=0$ and $z=3$.



              Thus, we got a value $6$.



              We'll prove that it'a maximal value.



              Indeed, let $xgeq ygeq z$.



              Thus, we need to prove that
              $$x-y+x-z+y-zleq6$$ or $$x-zleq3,$$ which is true because
              $$x-zleq xleq3.$$






              share|cite|improve this answer









              $endgroup$



              You need to make also one step only:



              Let $y=y=0$ and $z=3$.



              Thus, we got a value $6$.



              We'll prove that it'a maximal value.



              Indeed, let $xgeq ygeq z$.



              Thus, we need to prove that
              $$x-y+x-z+y-zleq6$$ or $$x-zleq3,$$ which is true because
              $$x-zleq xleq3.$$







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Dec 22 '18 at 5:31









              Michael RozenbergMichael Rozenberg

              107k1894198




              107k1894198























                  2












                  $begingroup$

                  Note that
                  $$
                  left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = M = const
                  $$

                  is a cylinder with axis $x=y=z$.



                  In fact
                  $$
                  eqalign{
                  & M = const = left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = cr
                  & = left| {left( {x - a} right) - left( {y - a} right)} right| + left| {left( {y - a} right) - left( {z - a} right)} right|
                  + left| {left( {z - a} right) - left( {x - a} right)} right| cr}
                  $$



                  Sq_Cylinder_1



                  The scheme tells you how you can determine the maximum within the given bounds.

                  For instance we see that, for $y=0$ we get
                  $$
                  eqalign{
                  & left{ matrix{
                  y = 0 hfill cr
                  0 le z,x hfill cr} right.quad Rightarrow quad left| x right| + left| z right| + left| {z - x} right| = 2left| z right| = Mquad Rightarrow cr
                  & Rightarrow quad max M = 6quad left| matrix{
                  ;0 le forall x le 3 hfill cr
                  ;y = 0 hfill cr
                  ;z = 3 hfill cr} right. cr}
                  $$

                  and the other sets of solutions will be symmetrical to this.






                  share|cite|improve this answer









                  $endgroup$


















                    2












                    $begingroup$

                    Note that
                    $$
                    left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = M = const
                    $$

                    is a cylinder with axis $x=y=z$.



                    In fact
                    $$
                    eqalign{
                    & M = const = left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = cr
                    & = left| {left( {x - a} right) - left( {y - a} right)} right| + left| {left( {y - a} right) - left( {z - a} right)} right|
                    + left| {left( {z - a} right) - left( {x - a} right)} right| cr}
                    $$



                    Sq_Cylinder_1



                    The scheme tells you how you can determine the maximum within the given bounds.

                    For instance we see that, for $y=0$ we get
                    $$
                    eqalign{
                    & left{ matrix{
                    y = 0 hfill cr
                    0 le z,x hfill cr} right.quad Rightarrow quad left| x right| + left| z right| + left| {z - x} right| = 2left| z right| = Mquad Rightarrow cr
                    & Rightarrow quad max M = 6quad left| matrix{
                    ;0 le forall x le 3 hfill cr
                    ;y = 0 hfill cr
                    ;z = 3 hfill cr} right. cr}
                    $$

                    and the other sets of solutions will be symmetrical to this.






                    share|cite|improve this answer









                    $endgroup$
















                      2












                      2








                      2





                      $begingroup$

                      Note that
                      $$
                      left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = M = const
                      $$

                      is a cylinder with axis $x=y=z$.



                      In fact
                      $$
                      eqalign{
                      & M = const = left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = cr
                      & = left| {left( {x - a} right) - left( {y - a} right)} right| + left| {left( {y - a} right) - left( {z - a} right)} right|
                      + left| {left( {z - a} right) - left( {x - a} right)} right| cr}
                      $$



                      Sq_Cylinder_1



                      The scheme tells you how you can determine the maximum within the given bounds.

                      For instance we see that, for $y=0$ we get
                      $$
                      eqalign{
                      & left{ matrix{
                      y = 0 hfill cr
                      0 le z,x hfill cr} right.quad Rightarrow quad left| x right| + left| z right| + left| {z - x} right| = 2left| z right| = Mquad Rightarrow cr
                      & Rightarrow quad max M = 6quad left| matrix{
                      ;0 le forall x le 3 hfill cr
                      ;y = 0 hfill cr
                      ;z = 3 hfill cr} right. cr}
                      $$

                      and the other sets of solutions will be symmetrical to this.






                      share|cite|improve this answer









                      $endgroup$



                      Note that
                      $$
                      left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = M = const
                      $$

                      is a cylinder with axis $x=y=z$.



                      In fact
                      $$
                      eqalign{
                      & M = const = left| {x - y} right| + left| {y - z} right| + left| {z - x} right| = cr
                      & = left| {left( {x - a} right) - left( {y - a} right)} right| + left| {left( {y - a} right) - left( {z - a} right)} right|
                      + left| {left( {z - a} right) - left( {x - a} right)} right| cr}
                      $$



                      Sq_Cylinder_1



                      The scheme tells you how you can determine the maximum within the given bounds.

                      For instance we see that, for $y=0$ we get
                      $$
                      eqalign{
                      & left{ matrix{
                      y = 0 hfill cr
                      0 le z,x hfill cr} right.quad Rightarrow quad left| x right| + left| z right| + left| {z - x} right| = 2left| z right| = Mquad Rightarrow cr
                      & Rightarrow quad max M = 6quad left| matrix{
                      ;0 le forall x le 3 hfill cr
                      ;y = 0 hfill cr
                      ;z = 3 hfill cr} right. cr}
                      $$

                      and the other sets of solutions will be symmetrical to this.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 22 '18 at 18:12









                      G CabG Cab

                      19.9k31340




                      19.9k31340























                          1












                          $begingroup$

                          You have $M=2z-2x $ as you have done
                          so we need to have $x=0$ in the maximal value of $M$
                          so $$Mle 2z$$
                          and $$zle 3$$
                          hence $$Mle 6$$






                          share|cite|improve this answer









                          $endgroup$


















                            1












                            $begingroup$

                            You have $M=2z-2x $ as you have done
                            so we need to have $x=0$ in the maximal value of $M$
                            so $$Mle 2z$$
                            and $$zle 3$$
                            hence $$Mle 6$$






                            share|cite|improve this answer









                            $endgroup$
















                              1












                              1








                              1





                              $begingroup$

                              You have $M=2z-2x $ as you have done
                              so we need to have $x=0$ in the maximal value of $M$
                              so $$Mle 2z$$
                              and $$zle 3$$
                              hence $$Mle 6$$






                              share|cite|improve this answer









                              $endgroup$



                              You have $M=2z-2x $ as you have done
                              so we need to have $x=0$ in the maximal value of $M$
                              so $$Mle 2z$$
                              and $$zle 3$$
                              hence $$Mle 6$$







                              share|cite|improve this answer












                              share|cite|improve this answer



                              share|cite|improve this answer










                              answered Dec 22 '18 at 16:24









                              user600785user600785

                              6810




                              6810























                                  1












                                  $begingroup$

                                  This solution might be a bit far reaching but the idea behind it may also help you in proving many other inequalities.



                                  The following facts give almost immediately the answer to your problem:





                                  • $M(x,y,z) = |x-y|+|y-z|+|z-x|$ is a convex function.


                                  • $Q = {(x,y,z) in mathbb{R}^3; | ; 0leq x,y,z leq 3 }$ is a compact (bounded and closed) convex set.

                                  • As $M$ is a continuous function on the compact set $Q$, $M$ has a maximum on $Q$.

                                  • Since $M$ and $Q$ are convex, the maximum is attained in an extremal point of $Q$ (the "corners" of the cube $Q$).

                                  • The corners of the cube $Q$ are $C = {(c_x,c_y,c_z) in mathbb{R}^3; | ; c_x,c_y,c_z in {0,3} } $


                                  By checking the values of $M$ at the corners you find the maximum at the corners $(3,0,0), (0,3,0), (0,0,3)$:
                                  $$max_Q M = max_C M = 6$$






                                  share|cite|improve this answer









                                  $endgroup$


















                                    1












                                    $begingroup$

                                    This solution might be a bit far reaching but the idea behind it may also help you in proving many other inequalities.



                                    The following facts give almost immediately the answer to your problem:





                                    • $M(x,y,z) = |x-y|+|y-z|+|z-x|$ is a convex function.


                                    • $Q = {(x,y,z) in mathbb{R}^3; | ; 0leq x,y,z leq 3 }$ is a compact (bounded and closed) convex set.

                                    • As $M$ is a continuous function on the compact set $Q$, $M$ has a maximum on $Q$.

                                    • Since $M$ and $Q$ are convex, the maximum is attained in an extremal point of $Q$ (the "corners" of the cube $Q$).

                                    • The corners of the cube $Q$ are $C = {(c_x,c_y,c_z) in mathbb{R}^3; | ; c_x,c_y,c_z in {0,3} } $


                                    By checking the values of $M$ at the corners you find the maximum at the corners $(3,0,0), (0,3,0), (0,0,3)$:
                                    $$max_Q M = max_C M = 6$$






                                    share|cite|improve this answer









                                    $endgroup$
















                                      1












                                      1








                                      1





                                      $begingroup$

                                      This solution might be a bit far reaching but the idea behind it may also help you in proving many other inequalities.



                                      The following facts give almost immediately the answer to your problem:





                                      • $M(x,y,z) = |x-y|+|y-z|+|z-x|$ is a convex function.


                                      • $Q = {(x,y,z) in mathbb{R}^3; | ; 0leq x,y,z leq 3 }$ is a compact (bounded and closed) convex set.

                                      • As $M$ is a continuous function on the compact set $Q$, $M$ has a maximum on $Q$.

                                      • Since $M$ and $Q$ are convex, the maximum is attained in an extremal point of $Q$ (the "corners" of the cube $Q$).

                                      • The corners of the cube $Q$ are $C = {(c_x,c_y,c_z) in mathbb{R}^3; | ; c_x,c_y,c_z in {0,3} } $


                                      By checking the values of $M$ at the corners you find the maximum at the corners $(3,0,0), (0,3,0), (0,0,3)$:
                                      $$max_Q M = max_C M = 6$$






                                      share|cite|improve this answer









                                      $endgroup$



                                      This solution might be a bit far reaching but the idea behind it may also help you in proving many other inequalities.



                                      The following facts give almost immediately the answer to your problem:





                                      • $M(x,y,z) = |x-y|+|y-z|+|z-x|$ is a convex function.


                                      • $Q = {(x,y,z) in mathbb{R}^3; | ; 0leq x,y,z leq 3 }$ is a compact (bounded and closed) convex set.

                                      • As $M$ is a continuous function on the compact set $Q$, $M$ has a maximum on $Q$.

                                      • Since $M$ and $Q$ are convex, the maximum is attained in an extremal point of $Q$ (the "corners" of the cube $Q$).

                                      • The corners of the cube $Q$ are $C = {(c_x,c_y,c_z) in mathbb{R}^3; | ; c_x,c_y,c_z in {0,3} } $


                                      By checking the values of $M$ at the corners you find the maximum at the corners $(3,0,0), (0,3,0), (0,0,3)$:
                                      $$max_Q M = max_C M = 6$$







                                      share|cite|improve this answer












                                      share|cite|improve this answer



                                      share|cite|improve this answer










                                      answered Dec 23 '18 at 4:00









                                      trancelocationtrancelocation

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