How to find the range of $y=frac{x^2+1}{x+1}$ without using derivative?












2












$begingroup$


The only thing I know with this equation is $y=frac{x^2+1}{x+1}=x+1-frac{2x}{x+1}$.



Maybe it can be solved by using inequality.










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  • $begingroup$
    math.stackexchange.com/questions/174905/…
    $endgroup$
    – lab bhattacharjee
    Jan 14 at 12:40
















2












$begingroup$


The only thing I know with this equation is $y=frac{x^2+1}{x+1}=x+1-frac{2x}{x+1}$.



Maybe it can be solved by using inequality.










share|cite|improve this question











$endgroup$












  • $begingroup$
    math.stackexchange.com/questions/174905/…
    $endgroup$
    – lab bhattacharjee
    Jan 14 at 12:40














2












2








2





$begingroup$


The only thing I know with this equation is $y=frac{x^2+1}{x+1}=x+1-frac{2x}{x+1}$.



Maybe it can be solved by using inequality.










share|cite|improve this question











$endgroup$




The only thing I know with this equation is $y=frac{x^2+1}{x+1}=x+1-frac{2x}{x+1}$.



Maybe it can be solved by using inequality.







inequality a.m.-g.m.-inequality






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 14 at 8:57









Michael Rozenberg

103k1891195




103k1891195










asked Jan 14 at 8:44









yuanming luoyuanming luo

737




737












  • $begingroup$
    math.stackexchange.com/questions/174905/…
    $endgroup$
    – lab bhattacharjee
    Jan 14 at 12:40


















  • $begingroup$
    math.stackexchange.com/questions/174905/…
    $endgroup$
    – lab bhattacharjee
    Jan 14 at 12:40
















$begingroup$
math.stackexchange.com/questions/174905/…
$endgroup$
– lab bhattacharjee
Jan 14 at 12:40




$begingroup$
math.stackexchange.com/questions/174905/…
$endgroup$
– lab bhattacharjee
Jan 14 at 12:40










4 Answers
4






active

oldest

votes


















2












$begingroup$

Does not exist.



Try $xrightarrow-1^-$.



For $x>-1$ by AM-GM we obtain:
$$frac{x^2+1}{x+1}=frac{x^2-1+2}{x+1}=x-1+frac{2}{x+1}=$$
$$=x+1+frac{2}{x+1}-2geq2sqrt{(x+1)cdotfrac{2}{x+1}}-2=2sqrt2-2.$$
The equality occurs for $x+1=frac{2}{x+1},$ which says that we got a minimal value and the local minimal value.



Thus, the range for $x>0$ it's $[2sqrt2-2,+infty).$



For $x<-1$ we can get the range by the similar way.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Sorry, I use the misunderstood word. It's a local maximum.
    $endgroup$
    – yuanming luo
    Jan 14 at 8:48










  • $begingroup$
    @yuanming luo I added something. See now.
    $endgroup$
    – Michael Rozenberg
    Jan 14 at 8:52










  • $begingroup$
    Thanks, I think I can apply the same tech for the negative side.
    $endgroup$
    – yuanming luo
    Jan 14 at 8:55










  • $begingroup$
    @yuanming luo Yes, of course!
    $endgroup$
    – Michael Rozenberg
    Jan 14 at 8:56










  • $begingroup$
    How can I know the result found by AM-GM is the lower bound at the first quadrant?
    $endgroup$
    – yuanming luo
    Jan 16 at 0:13



















3












$begingroup$

The equation $$y={x^2+1over x+1}=ato x^2-ax-(a-1)=0$$for $xne -1$ has a real root if and only if $$Deltage0 iff a^2+4a-4ge 0iff age 2sqrt 2-2text{ or }ale -2sqrt 2-2$$in that case, the root is $ne -1$ automatically (if not, it leads to the impossible non-equality $a-1=a+1$), Therefore the range simply is $$(-infty,-2sqrt 2-2] bigcup [2sqrt 2-2,infty)$$






share|cite|improve this answer









$endgroup$





















    1












    $begingroup$

    Out of the given answers, one simple way is to get the $x$ in terms of $y$ and solve it :)



    $y=frac{x^2+1}{x+1}$
    $$Rightarrow yx + y = x^2 +1 Rightarrow -x^2 + yx +(y-1)=0 Rightarrow x = frac{-y pmsqrt{y^2 + 4(y-1)}}{-2} Rightarrow frac{-y pm sqrt{(y-(-2-sqrt5))(y - (sqrt5-2))}}{-2}$$



    The Lowest Value so will be obtained will be when the value under square root = 0, then in that case



    $$ = frac{-sqrt5+2}{-2}$$



    Now plug that value in the expression for $x$ Gives the minimal value of $y$:






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      Since $$lim_{xto-infty} frac{x^2+1}{x+1} = -infty,$$



      the minimum does not exist.






      share|cite|improve this answer









      $endgroup$













      • $begingroup$
        sorry, I use the misunderstood word. It's a local maximum.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:47











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      4 Answers
      4






      active

      oldest

      votes








      4 Answers
      4






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      2












      $begingroup$

      Does not exist.



      Try $xrightarrow-1^-$.



      For $x>-1$ by AM-GM we obtain:
      $$frac{x^2+1}{x+1}=frac{x^2-1+2}{x+1}=x-1+frac{2}{x+1}=$$
      $$=x+1+frac{2}{x+1}-2geq2sqrt{(x+1)cdotfrac{2}{x+1}}-2=2sqrt2-2.$$
      The equality occurs for $x+1=frac{2}{x+1},$ which says that we got a minimal value and the local minimal value.



      Thus, the range for $x>0$ it's $[2sqrt2-2,+infty).$



      For $x<-1$ we can get the range by the similar way.






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        Sorry, I use the misunderstood word. It's a local maximum.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:48










      • $begingroup$
        @yuanming luo I added something. See now.
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:52










      • $begingroup$
        Thanks, I think I can apply the same tech for the negative side.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:55










      • $begingroup$
        @yuanming luo Yes, of course!
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:56










      • $begingroup$
        How can I know the result found by AM-GM is the lower bound at the first quadrant?
        $endgroup$
        – yuanming luo
        Jan 16 at 0:13
















      2












      $begingroup$

      Does not exist.



      Try $xrightarrow-1^-$.



      For $x>-1$ by AM-GM we obtain:
      $$frac{x^2+1}{x+1}=frac{x^2-1+2}{x+1}=x-1+frac{2}{x+1}=$$
      $$=x+1+frac{2}{x+1}-2geq2sqrt{(x+1)cdotfrac{2}{x+1}}-2=2sqrt2-2.$$
      The equality occurs for $x+1=frac{2}{x+1},$ which says that we got a minimal value and the local minimal value.



      Thus, the range for $x>0$ it's $[2sqrt2-2,+infty).$



      For $x<-1$ we can get the range by the similar way.






      share|cite|improve this answer











      $endgroup$













      • $begingroup$
        Sorry, I use the misunderstood word. It's a local maximum.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:48










      • $begingroup$
        @yuanming luo I added something. See now.
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:52










      • $begingroup$
        Thanks, I think I can apply the same tech for the negative side.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:55










      • $begingroup$
        @yuanming luo Yes, of course!
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:56










      • $begingroup$
        How can I know the result found by AM-GM is the lower bound at the first quadrant?
        $endgroup$
        – yuanming luo
        Jan 16 at 0:13














      2












      2








      2





      $begingroup$

      Does not exist.



      Try $xrightarrow-1^-$.



      For $x>-1$ by AM-GM we obtain:
      $$frac{x^2+1}{x+1}=frac{x^2-1+2}{x+1}=x-1+frac{2}{x+1}=$$
      $$=x+1+frac{2}{x+1}-2geq2sqrt{(x+1)cdotfrac{2}{x+1}}-2=2sqrt2-2.$$
      The equality occurs for $x+1=frac{2}{x+1},$ which says that we got a minimal value and the local minimal value.



      Thus, the range for $x>0$ it's $[2sqrt2-2,+infty).$



      For $x<-1$ we can get the range by the similar way.






      share|cite|improve this answer











      $endgroup$



      Does not exist.



      Try $xrightarrow-1^-$.



      For $x>-1$ by AM-GM we obtain:
      $$frac{x^2+1}{x+1}=frac{x^2-1+2}{x+1}=x-1+frac{2}{x+1}=$$
      $$=x+1+frac{2}{x+1}-2geq2sqrt{(x+1)cdotfrac{2}{x+1}}-2=2sqrt2-2.$$
      The equality occurs for $x+1=frac{2}{x+1},$ which says that we got a minimal value and the local minimal value.



      Thus, the range for $x>0$ it's $[2sqrt2-2,+infty).$



      For $x<-1$ we can get the range by the similar way.







      share|cite|improve this answer














      share|cite|improve this answer



      share|cite|improve this answer








      edited Jan 14 at 8:55

























      answered Jan 14 at 8:46









      Michael RozenbergMichael Rozenberg

      103k1891195




      103k1891195












      • $begingroup$
        Sorry, I use the misunderstood word. It's a local maximum.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:48










      • $begingroup$
        @yuanming luo I added something. See now.
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:52










      • $begingroup$
        Thanks, I think I can apply the same tech for the negative side.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:55










      • $begingroup$
        @yuanming luo Yes, of course!
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:56










      • $begingroup$
        How can I know the result found by AM-GM is the lower bound at the first quadrant?
        $endgroup$
        – yuanming luo
        Jan 16 at 0:13


















      • $begingroup$
        Sorry, I use the misunderstood word. It's a local maximum.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:48










      • $begingroup$
        @yuanming luo I added something. See now.
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:52










      • $begingroup$
        Thanks, I think I can apply the same tech for the negative side.
        $endgroup$
        – yuanming luo
        Jan 14 at 8:55










      • $begingroup$
        @yuanming luo Yes, of course!
        $endgroup$
        – Michael Rozenberg
        Jan 14 at 8:56










      • $begingroup$
        How can I know the result found by AM-GM is the lower bound at the first quadrant?
        $endgroup$
        – yuanming luo
        Jan 16 at 0:13
















      $begingroup$
      Sorry, I use the misunderstood word. It's a local maximum.
      $endgroup$
      – yuanming luo
      Jan 14 at 8:48




      $begingroup$
      Sorry, I use the misunderstood word. It's a local maximum.
      $endgroup$
      – yuanming luo
      Jan 14 at 8:48












      $begingroup$
      @yuanming luo I added something. See now.
      $endgroup$
      – Michael Rozenberg
      Jan 14 at 8:52




      $begingroup$
      @yuanming luo I added something. See now.
      $endgroup$
      – Michael Rozenberg
      Jan 14 at 8:52












      $begingroup$
      Thanks, I think I can apply the same tech for the negative side.
      $endgroup$
      – yuanming luo
      Jan 14 at 8:55




      $begingroup$
      Thanks, I think I can apply the same tech for the negative side.
      $endgroup$
      – yuanming luo
      Jan 14 at 8:55












      $begingroup$
      @yuanming luo Yes, of course!
      $endgroup$
      – Michael Rozenberg
      Jan 14 at 8:56




      $begingroup$
      @yuanming luo Yes, of course!
      $endgroup$
      – Michael Rozenberg
      Jan 14 at 8:56












      $begingroup$
      How can I know the result found by AM-GM is the lower bound at the first quadrant?
      $endgroup$
      – yuanming luo
      Jan 16 at 0:13




      $begingroup$
      How can I know the result found by AM-GM is the lower bound at the first quadrant?
      $endgroup$
      – yuanming luo
      Jan 16 at 0:13











      3












      $begingroup$

      The equation $$y={x^2+1over x+1}=ato x^2-ax-(a-1)=0$$for $xne -1$ has a real root if and only if $$Deltage0 iff a^2+4a-4ge 0iff age 2sqrt 2-2text{ or }ale -2sqrt 2-2$$in that case, the root is $ne -1$ automatically (if not, it leads to the impossible non-equality $a-1=a+1$), Therefore the range simply is $$(-infty,-2sqrt 2-2] bigcup [2sqrt 2-2,infty)$$






      share|cite|improve this answer









      $endgroup$


















        3












        $begingroup$

        The equation $$y={x^2+1over x+1}=ato x^2-ax-(a-1)=0$$for $xne -1$ has a real root if and only if $$Deltage0 iff a^2+4a-4ge 0iff age 2sqrt 2-2text{ or }ale -2sqrt 2-2$$in that case, the root is $ne -1$ automatically (if not, it leads to the impossible non-equality $a-1=a+1$), Therefore the range simply is $$(-infty,-2sqrt 2-2] bigcup [2sqrt 2-2,infty)$$






        share|cite|improve this answer









        $endgroup$
















          3












          3








          3





          $begingroup$

          The equation $$y={x^2+1over x+1}=ato x^2-ax-(a-1)=0$$for $xne -1$ has a real root if and only if $$Deltage0 iff a^2+4a-4ge 0iff age 2sqrt 2-2text{ or }ale -2sqrt 2-2$$in that case, the root is $ne -1$ automatically (if not, it leads to the impossible non-equality $a-1=a+1$), Therefore the range simply is $$(-infty,-2sqrt 2-2] bigcup [2sqrt 2-2,infty)$$






          share|cite|improve this answer









          $endgroup$



          The equation $$y={x^2+1over x+1}=ato x^2-ax-(a-1)=0$$for $xne -1$ has a real root if and only if $$Deltage0 iff a^2+4a-4ge 0iff age 2sqrt 2-2text{ or }ale -2sqrt 2-2$$in that case, the root is $ne -1$ automatically (if not, it leads to the impossible non-equality $a-1=a+1$), Therefore the range simply is $$(-infty,-2sqrt 2-2] bigcup [2sqrt 2-2,infty)$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 14 at 9:31









          Mostafa AyazMostafa Ayaz

          15.6k3939




          15.6k3939























              1












              $begingroup$

              Out of the given answers, one simple way is to get the $x$ in terms of $y$ and solve it :)



              $y=frac{x^2+1}{x+1}$
              $$Rightarrow yx + y = x^2 +1 Rightarrow -x^2 + yx +(y-1)=0 Rightarrow x = frac{-y pmsqrt{y^2 + 4(y-1)}}{-2} Rightarrow frac{-y pm sqrt{(y-(-2-sqrt5))(y - (sqrt5-2))}}{-2}$$



              The Lowest Value so will be obtained will be when the value under square root = 0, then in that case



              $$ = frac{-sqrt5+2}{-2}$$



              Now plug that value in the expression for $x$ Gives the minimal value of $y$:






              share|cite|improve this answer









              $endgroup$


















                1












                $begingroup$

                Out of the given answers, one simple way is to get the $x$ in terms of $y$ and solve it :)



                $y=frac{x^2+1}{x+1}$
                $$Rightarrow yx + y = x^2 +1 Rightarrow -x^2 + yx +(y-1)=0 Rightarrow x = frac{-y pmsqrt{y^2 + 4(y-1)}}{-2} Rightarrow frac{-y pm sqrt{(y-(-2-sqrt5))(y - (sqrt5-2))}}{-2}$$



                The Lowest Value so will be obtained will be when the value under square root = 0, then in that case



                $$ = frac{-sqrt5+2}{-2}$$



                Now plug that value in the expression for $x$ Gives the minimal value of $y$:






                share|cite|improve this answer









                $endgroup$
















                  1












                  1








                  1





                  $begingroup$

                  Out of the given answers, one simple way is to get the $x$ in terms of $y$ and solve it :)



                  $y=frac{x^2+1}{x+1}$
                  $$Rightarrow yx + y = x^2 +1 Rightarrow -x^2 + yx +(y-1)=0 Rightarrow x = frac{-y pmsqrt{y^2 + 4(y-1)}}{-2} Rightarrow frac{-y pm sqrt{(y-(-2-sqrt5))(y - (sqrt5-2))}}{-2}$$



                  The Lowest Value so will be obtained will be when the value under square root = 0, then in that case



                  $$ = frac{-sqrt5+2}{-2}$$



                  Now plug that value in the expression for $x$ Gives the minimal value of $y$:






                  share|cite|improve this answer









                  $endgroup$



                  Out of the given answers, one simple way is to get the $x$ in terms of $y$ and solve it :)



                  $y=frac{x^2+1}{x+1}$
                  $$Rightarrow yx + y = x^2 +1 Rightarrow -x^2 + yx +(y-1)=0 Rightarrow x = frac{-y pmsqrt{y^2 + 4(y-1)}}{-2} Rightarrow frac{-y pm sqrt{(y-(-2-sqrt5))(y - (sqrt5-2))}}{-2}$$



                  The Lowest Value so will be obtained will be when the value under square root = 0, then in that case



                  $$ = frac{-sqrt5+2}{-2}$$



                  Now plug that value in the expression for $x$ Gives the minimal value of $y$:







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Jan 14 at 10:47









                  Abhas Kumar SinhaAbhas Kumar Sinha

                  18715




                  18715























                      0












                      $begingroup$

                      Since $$lim_{xto-infty} frac{x^2+1}{x+1} = -infty,$$



                      the minimum does not exist.






                      share|cite|improve this answer









                      $endgroup$













                      • $begingroup$
                        sorry, I use the misunderstood word. It's a local maximum.
                        $endgroup$
                        – yuanming luo
                        Jan 14 at 8:47
















                      0












                      $begingroup$

                      Since $$lim_{xto-infty} frac{x^2+1}{x+1} = -infty,$$



                      the minimum does not exist.






                      share|cite|improve this answer









                      $endgroup$













                      • $begingroup$
                        sorry, I use the misunderstood word. It's a local maximum.
                        $endgroup$
                        – yuanming luo
                        Jan 14 at 8:47














                      0












                      0








                      0





                      $begingroup$

                      Since $$lim_{xto-infty} frac{x^2+1}{x+1} = -infty,$$



                      the minimum does not exist.






                      share|cite|improve this answer









                      $endgroup$



                      Since $$lim_{xto-infty} frac{x^2+1}{x+1} = -infty,$$



                      the minimum does not exist.







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Jan 14 at 8:46









                      5xum5xum

                      90.8k394161




                      90.8k394161












                      • $begingroup$
                        sorry, I use the misunderstood word. It's a local maximum.
                        $endgroup$
                        – yuanming luo
                        Jan 14 at 8:47


















                      • $begingroup$
                        sorry, I use the misunderstood word. It's a local maximum.
                        $endgroup$
                        – yuanming luo
                        Jan 14 at 8:47
















                      $begingroup$
                      sorry, I use the misunderstood word. It's a local maximum.
                      $endgroup$
                      – yuanming luo
                      Jan 14 at 8:47




                      $begingroup$
                      sorry, I use the misunderstood word. It's a local maximum.
                      $endgroup$
                      – yuanming luo
                      Jan 14 at 8:47


















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