Can't find $lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x$

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I'm trying to solve this limit. Wolfram showed, that there's no limit, but I can clearly see that the limit exists from graph. Tried L'Hopital's rule, but didn't get any further.
$$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x$$
I don't know which method should I use










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  • what limit do you see on the graph?
    – Rushabh Mehta
    Oct 3 at 21:29










  • @RushabhMehta, I made graph in google, so I can't say exactly, but it's around 0.4
    – Narek Maloyan
    Oct 3 at 21:32











  • Could you link the graph in the comments? I am intrigued
    – Rushabh Mehta
    Oct 3 at 21:32










  • @RushabhMehta, it should show google.com/…
    – Narek Maloyan
    Oct 3 at 21:34











  • Well, Google is being really weird I guess. That's not even close to the actual graph of the function. This is closer.
    – Rushabh Mehta
    Oct 3 at 21:37















up vote
1
down vote

favorite












I'm trying to solve this limit. Wolfram showed, that there's no limit, but I can clearly see that the limit exists from graph. Tried L'Hopital's rule, but didn't get any further.
$$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x$$
I don't know which method should I use










share|cite|improve this question























  • what limit do you see on the graph?
    – Rushabh Mehta
    Oct 3 at 21:29










  • @RushabhMehta, I made graph in google, so I can't say exactly, but it's around 0.4
    – Narek Maloyan
    Oct 3 at 21:32











  • Could you link the graph in the comments? I am intrigued
    – Rushabh Mehta
    Oct 3 at 21:32










  • @RushabhMehta, it should show google.com/…
    – Narek Maloyan
    Oct 3 at 21:34











  • Well, Google is being really weird I guess. That's not even close to the actual graph of the function. This is closer.
    – Rushabh Mehta
    Oct 3 at 21:37













up vote
1
down vote

favorite









up vote
1
down vote

favorite











I'm trying to solve this limit. Wolfram showed, that there's no limit, but I can clearly see that the limit exists from graph. Tried L'Hopital's rule, but didn't get any further.
$$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x$$
I don't know which method should I use










share|cite|improve this question















I'm trying to solve this limit. Wolfram showed, that there's no limit, but I can clearly see that the limit exists from graph. Tried L'Hopital's rule, but didn't get any further.
$$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x$$
I don't know which method should I use







limits






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edited Oct 4 at 0:00









Asaf Karagila♦

296k32412739




296k32412739










asked Oct 3 at 21:24









Narek Maloyan

477312




477312











  • what limit do you see on the graph?
    – Rushabh Mehta
    Oct 3 at 21:29










  • @RushabhMehta, I made graph in google, so I can't say exactly, but it's around 0.4
    – Narek Maloyan
    Oct 3 at 21:32











  • Could you link the graph in the comments? I am intrigued
    – Rushabh Mehta
    Oct 3 at 21:32










  • @RushabhMehta, it should show google.com/…
    – Narek Maloyan
    Oct 3 at 21:34











  • Well, Google is being really weird I guess. That's not even close to the actual graph of the function. This is closer.
    – Rushabh Mehta
    Oct 3 at 21:37

















  • what limit do you see on the graph?
    – Rushabh Mehta
    Oct 3 at 21:29










  • @RushabhMehta, I made graph in google, so I can't say exactly, but it's around 0.4
    – Narek Maloyan
    Oct 3 at 21:32











  • Could you link the graph in the comments? I am intrigued
    – Rushabh Mehta
    Oct 3 at 21:32










  • @RushabhMehta, it should show google.com/…
    – Narek Maloyan
    Oct 3 at 21:34











  • Well, Google is being really weird I guess. That's not even close to the actual graph of the function. This is closer.
    – Rushabh Mehta
    Oct 3 at 21:37
















what limit do you see on the graph?
– Rushabh Mehta
Oct 3 at 21:29




what limit do you see on the graph?
– Rushabh Mehta
Oct 3 at 21:29












@RushabhMehta, I made graph in google, so I can't say exactly, but it's around 0.4
– Narek Maloyan
Oct 3 at 21:32





@RushabhMehta, I made graph in google, so I can't say exactly, but it's around 0.4
– Narek Maloyan
Oct 3 at 21:32













Could you link the graph in the comments? I am intrigued
– Rushabh Mehta
Oct 3 at 21:32




Could you link the graph in the comments? I am intrigued
– Rushabh Mehta
Oct 3 at 21:32












@RushabhMehta, it should show google.com/…
– Narek Maloyan
Oct 3 at 21:34





@RushabhMehta, it should show google.com/…
– Narek Maloyan
Oct 3 at 21:34













Well, Google is being really weird I guess. That's not even close to the actual graph of the function. This is closer.
– Rushabh Mehta
Oct 3 at 21:37





Well, Google is being really weird I guess. That's not even close to the actual graph of the function. This is closer.
– Rushabh Mehta
Oct 3 at 21:37











3 Answers
3






active

oldest

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up vote
3
down vote



accepted










Set $x=-1+h;$ ($hto 0$) and use the binomial approximation:




  • $sqrt[3]1+2(-1+h)+1=1-sqrt[3]1-2h=1-bigl(1-frac23 h+o(h)bigl)=frac23 h+o(h)$,


  • $sqrt2+(-1+h)-1+h=sqrt1+h-1+h=1+frac12h+o(h)-1+h=frac32h+o(h),$
    so $;dfracsqrt[3]1+2x+1sqrt2+x + x=dfracfrac23 h+o(h)frac32h+o(h))=dfracfrac23+o(1)frac32+o(1)=dfrac49+o(1).$





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    up vote
    5
    down vote













    Recall the formulas:
    $$
    beginalign
    a^2 -b^2 &= (a - b)(a + b)\
    a^3 + b^3 &= (a + b)(a^2 - ab + b^2)
    endalign
    $$

    Using them we can get the following:
    $$
    beginalign
    sqrt[3]1+2x + 1 &=
    frac1 + 2x + 1sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 =
    frac2(x + 1)sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 \
    sqrt2+x + x &=
    frac2 + x - x^2sqrt2+x - x =
    -frac(x + 1)(x - 2)sqrt2+x - x
    endalign
    $$

    Therefore, your limit is equal to
    $$
    lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x =
    lim_xto-1 -frac2(sqrt2+x - x)(x - 2)(sqrt[3]1+2x^2 - sqrt[3]1+2x + 1),
    $$

    which is pretty straightforward to compute.






    share|cite|improve this answer




















    • That's beautiful
      – Narek Maloyan
      Oct 3 at 21:57

















    up vote
    3
    down vote













    L'Hospital works:



    $$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x=lim_xto-1 fracdfrac23(sqrt[3]1+2x)^2dfrac12sqrt2+x + 1=frac49.$$






    share|cite|improve this answer




















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      3 Answers
      3






      active

      oldest

      votes








      3 Answers
      3






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes








      up vote
      3
      down vote



      accepted










      Set $x=-1+h;$ ($hto 0$) and use the binomial approximation:




      • $sqrt[3]1+2(-1+h)+1=1-sqrt[3]1-2h=1-bigl(1-frac23 h+o(h)bigl)=frac23 h+o(h)$,


      • $sqrt2+(-1+h)-1+h=sqrt1+h-1+h=1+frac12h+o(h)-1+h=frac32h+o(h),$
        so $;dfracsqrt[3]1+2x+1sqrt2+x + x=dfracfrac23 h+o(h)frac32h+o(h))=dfracfrac23+o(1)frac32+o(1)=dfrac49+o(1).$





      share|cite|improve this answer
























        up vote
        3
        down vote



        accepted










        Set $x=-1+h;$ ($hto 0$) and use the binomial approximation:




        • $sqrt[3]1+2(-1+h)+1=1-sqrt[3]1-2h=1-bigl(1-frac23 h+o(h)bigl)=frac23 h+o(h)$,


        • $sqrt2+(-1+h)-1+h=sqrt1+h-1+h=1+frac12h+o(h)-1+h=frac32h+o(h),$
          so $;dfracsqrt[3]1+2x+1sqrt2+x + x=dfracfrac23 h+o(h)frac32h+o(h))=dfracfrac23+o(1)frac32+o(1)=dfrac49+o(1).$





        share|cite|improve this answer






















          up vote
          3
          down vote



          accepted







          up vote
          3
          down vote



          accepted






          Set $x=-1+h;$ ($hto 0$) and use the binomial approximation:




          • $sqrt[3]1+2(-1+h)+1=1-sqrt[3]1-2h=1-bigl(1-frac23 h+o(h)bigl)=frac23 h+o(h)$,


          • $sqrt2+(-1+h)-1+h=sqrt1+h-1+h=1+frac12h+o(h)-1+h=frac32h+o(h),$
            so $;dfracsqrt[3]1+2x+1sqrt2+x + x=dfracfrac23 h+o(h)frac32h+o(h))=dfracfrac23+o(1)frac32+o(1)=dfrac49+o(1).$





          share|cite|improve this answer












          Set $x=-1+h;$ ($hto 0$) and use the binomial approximation:




          • $sqrt[3]1+2(-1+h)+1=1-sqrt[3]1-2h=1-bigl(1-frac23 h+o(h)bigl)=frac23 h+o(h)$,


          • $sqrt2+(-1+h)-1+h=sqrt1+h-1+h=1+frac12h+o(h)-1+h=frac32h+o(h),$
            so $;dfracsqrt[3]1+2x+1sqrt2+x + x=dfracfrac23 h+o(h)frac32h+o(h))=dfracfrac23+o(1)frac32+o(1)=dfrac49+o(1).$






          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Oct 3 at 21:39









          Bernard

          113k636104




          113k636104




















              up vote
              5
              down vote













              Recall the formulas:
              $$
              beginalign
              a^2 -b^2 &= (a - b)(a + b)\
              a^3 + b^3 &= (a + b)(a^2 - ab + b^2)
              endalign
              $$

              Using them we can get the following:
              $$
              beginalign
              sqrt[3]1+2x + 1 &=
              frac1 + 2x + 1sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 =
              frac2(x + 1)sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 \
              sqrt2+x + x &=
              frac2 + x - x^2sqrt2+x - x =
              -frac(x + 1)(x - 2)sqrt2+x - x
              endalign
              $$

              Therefore, your limit is equal to
              $$
              lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x =
              lim_xto-1 -frac2(sqrt2+x - x)(x - 2)(sqrt[3]1+2x^2 - sqrt[3]1+2x + 1),
              $$

              which is pretty straightforward to compute.






              share|cite|improve this answer




















              • That's beautiful
                – Narek Maloyan
                Oct 3 at 21:57














              up vote
              5
              down vote













              Recall the formulas:
              $$
              beginalign
              a^2 -b^2 &= (a - b)(a + b)\
              a^3 + b^3 &= (a + b)(a^2 - ab + b^2)
              endalign
              $$

              Using them we can get the following:
              $$
              beginalign
              sqrt[3]1+2x + 1 &=
              frac1 + 2x + 1sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 =
              frac2(x + 1)sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 \
              sqrt2+x + x &=
              frac2 + x - x^2sqrt2+x - x =
              -frac(x + 1)(x - 2)sqrt2+x - x
              endalign
              $$

              Therefore, your limit is equal to
              $$
              lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x =
              lim_xto-1 -frac2(sqrt2+x - x)(x - 2)(sqrt[3]1+2x^2 - sqrt[3]1+2x + 1),
              $$

              which is pretty straightforward to compute.






              share|cite|improve this answer




















              • That's beautiful
                – Narek Maloyan
                Oct 3 at 21:57












              up vote
              5
              down vote










              up vote
              5
              down vote









              Recall the formulas:
              $$
              beginalign
              a^2 -b^2 &= (a - b)(a + b)\
              a^3 + b^3 &= (a + b)(a^2 - ab + b^2)
              endalign
              $$

              Using them we can get the following:
              $$
              beginalign
              sqrt[3]1+2x + 1 &=
              frac1 + 2x + 1sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 =
              frac2(x + 1)sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 \
              sqrt2+x + x &=
              frac2 + x - x^2sqrt2+x - x =
              -frac(x + 1)(x - 2)sqrt2+x - x
              endalign
              $$

              Therefore, your limit is equal to
              $$
              lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x =
              lim_xto-1 -frac2(sqrt2+x - x)(x - 2)(sqrt[3]1+2x^2 - sqrt[3]1+2x + 1),
              $$

              which is pretty straightforward to compute.






              share|cite|improve this answer












              Recall the formulas:
              $$
              beginalign
              a^2 -b^2 &= (a - b)(a + b)\
              a^3 + b^3 &= (a + b)(a^2 - ab + b^2)
              endalign
              $$

              Using them we can get the following:
              $$
              beginalign
              sqrt[3]1+2x + 1 &=
              frac1 + 2x + 1sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 =
              frac2(x + 1)sqrt[3]1+2x^2 - sqrt[3]1+2x + 1 \
              sqrt2+x + x &=
              frac2 + x - x^2sqrt2+x - x =
              -frac(x + 1)(x - 2)sqrt2+x - x
              endalign
              $$

              Therefore, your limit is equal to
              $$
              lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x =
              lim_xto-1 -frac2(sqrt2+x - x)(x - 2)(sqrt[3]1+2x^2 - sqrt[3]1+2x + 1),
              $$

              which is pretty straightforward to compute.







              share|cite|improve this answer












              share|cite|improve this answer



              share|cite|improve this answer










              answered Oct 3 at 21:55









              mwt

              913116




              913116











              • That's beautiful
                – Narek Maloyan
                Oct 3 at 21:57
















              • That's beautiful
                – Narek Maloyan
                Oct 3 at 21:57















              That's beautiful
              – Narek Maloyan
              Oct 3 at 21:57




              That's beautiful
              – Narek Maloyan
              Oct 3 at 21:57










              up vote
              3
              down vote













              L'Hospital works:



              $$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x=lim_xto-1 fracdfrac23(sqrt[3]1+2x)^2dfrac12sqrt2+x + 1=frac49.$$






              share|cite|improve this answer
























                up vote
                3
                down vote













                L'Hospital works:



                $$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x=lim_xto-1 fracdfrac23(sqrt[3]1+2x)^2dfrac12sqrt2+x + 1=frac49.$$






                share|cite|improve this answer






















                  up vote
                  3
                  down vote










                  up vote
                  3
                  down vote









                  L'Hospital works:



                  $$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x=lim_xto-1 fracdfrac23(sqrt[3]1+2x)^2dfrac12sqrt2+x + 1=frac49.$$






                  share|cite|improve this answer












                  L'Hospital works:



                  $$lim_xto-1 fracsqrt[3]1+2x+1sqrt2+x + x=lim_xto-1 fracdfrac23(sqrt[3]1+2x)^2dfrac12sqrt2+x + 1=frac49.$$







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Oct 3 at 22:08









                  Yves Daoust

                  117k667213




                  117k667213



























                       

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