Showing that $y = phi over phi -pi cdot sinphi$ tends to $-pi$ as $phi to pi$ without using limits [on hold]

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1
down vote

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I'm trying to manually plot a parametric function:



$$
r=fracphiphi - pi
$$



I've translated the above to Сartesian coordinate system and found zeros for $x$ and $y$ axes.



$$
cases
x = rcosphi = fracphiphi - picdot cosphi\
y = rsinphi = fracphiphi - picdot sinphi

$$



Then i wanted to find the value of $x$ and $y$ as $phi to pi$. It's clear that $x$ tends to $pm infty$ depending from which side $phi$ approaches $pi$ since $cospi = -1$. But it's not clear how to deal with the value of $y$ since it's $y =pm inftycdot0$.



I've found the limit using L'opitals rule, but the problem is taken from a precalculus section of the book i'm solving. I am not supposed to use limits or derivatives.



I've already spent a couple of hours pondering on that, but that doesn't seem very obvious to me.




So now I'm wondering how could I find that limit in a precalculus manner?




Below is the graph in case someone is curious what it's really about:



polarplot







share|cite|improve this question











put on hold as unclear what you're asking by Xander Henderson, Shailesh, Mostafa Ayaz, Lord Shark the Unknown, John B Aug 3 at 9:28


Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.










  • 3




    You want to compute a limit without using limits? I am very confused about what it is that you are trying to accomplish...
    – Xander Henderson
    Aug 2 at 16:51






  • 1




    @XanderHenderson I need to know a value which is approached by $y$ as $phi to pi$, otherwise i can not build the plot.
    – roman
    Aug 2 at 16:57






  • 2




    Yes, but that involves computing a limit! Indeed, the answer that you have accepted requires computation of a limit. I don't understand what you are asking...
    – Xander Henderson
    Aug 2 at 17:01






  • 1




    @XanderHenderson I've accepted the below, because $sinx over x = 1$ can be shown geometrically without involving derivatives, and that's what i actually needed. I understand it's confusing to see "find a limit without a limit", but basically i intended to say "find a value of $y$" which is know as limit in calculus
    – roman
    Aug 2 at 17:10











  • It can be shown without derivatives, but it cannot be shown without limits, which is why I am confused.
    – Xander Henderson
    Aug 2 at 18:09














up vote
1
down vote

favorite












I'm trying to manually plot a parametric function:



$$
r=fracphiphi - pi
$$



I've translated the above to Сartesian coordinate system and found zeros for $x$ and $y$ axes.



$$
cases
x = rcosphi = fracphiphi - picdot cosphi\
y = rsinphi = fracphiphi - picdot sinphi

$$



Then i wanted to find the value of $x$ and $y$ as $phi to pi$. It's clear that $x$ tends to $pm infty$ depending from which side $phi$ approaches $pi$ since $cospi = -1$. But it's not clear how to deal with the value of $y$ since it's $y =pm inftycdot0$.



I've found the limit using L'opitals rule, but the problem is taken from a precalculus section of the book i'm solving. I am not supposed to use limits or derivatives.



I've already spent a couple of hours pondering on that, but that doesn't seem very obvious to me.




So now I'm wondering how could I find that limit in a precalculus manner?




Below is the graph in case someone is curious what it's really about:



polarplot







share|cite|improve this question











put on hold as unclear what you're asking by Xander Henderson, Shailesh, Mostafa Ayaz, Lord Shark the Unknown, John B Aug 3 at 9:28


Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.










  • 3




    You want to compute a limit without using limits? I am very confused about what it is that you are trying to accomplish...
    – Xander Henderson
    Aug 2 at 16:51






  • 1




    @XanderHenderson I need to know a value which is approached by $y$ as $phi to pi$, otherwise i can not build the plot.
    – roman
    Aug 2 at 16:57






  • 2




    Yes, but that involves computing a limit! Indeed, the answer that you have accepted requires computation of a limit. I don't understand what you are asking...
    – Xander Henderson
    Aug 2 at 17:01






  • 1




    @XanderHenderson I've accepted the below, because $sinx over x = 1$ can be shown geometrically without involving derivatives, and that's what i actually needed. I understand it's confusing to see "find a limit without a limit", but basically i intended to say "find a value of $y$" which is know as limit in calculus
    – roman
    Aug 2 at 17:10











  • It can be shown without derivatives, but it cannot be shown without limits, which is why I am confused.
    – Xander Henderson
    Aug 2 at 18:09












up vote
1
down vote

favorite









up vote
1
down vote

favorite











I'm trying to manually plot a parametric function:



$$
r=fracphiphi - pi
$$



I've translated the above to Сartesian coordinate system and found zeros for $x$ and $y$ axes.



$$
cases
x = rcosphi = fracphiphi - picdot cosphi\
y = rsinphi = fracphiphi - picdot sinphi

$$



Then i wanted to find the value of $x$ and $y$ as $phi to pi$. It's clear that $x$ tends to $pm infty$ depending from which side $phi$ approaches $pi$ since $cospi = -1$. But it's not clear how to deal with the value of $y$ since it's $y =pm inftycdot0$.



I've found the limit using L'opitals rule, but the problem is taken from a precalculus section of the book i'm solving. I am not supposed to use limits or derivatives.



I've already spent a couple of hours pondering on that, but that doesn't seem very obvious to me.




So now I'm wondering how could I find that limit in a precalculus manner?




Below is the graph in case someone is curious what it's really about:



polarplot







share|cite|improve this question











I'm trying to manually plot a parametric function:



$$
r=fracphiphi - pi
$$



I've translated the above to Сartesian coordinate system and found zeros for $x$ and $y$ axes.



$$
cases
x = rcosphi = fracphiphi - picdot cosphi\
y = rsinphi = fracphiphi - picdot sinphi

$$



Then i wanted to find the value of $x$ and $y$ as $phi to pi$. It's clear that $x$ tends to $pm infty$ depending from which side $phi$ approaches $pi$ since $cospi = -1$. But it's not clear how to deal with the value of $y$ since it's $y =pm inftycdot0$.



I've found the limit using L'opitals rule, but the problem is taken from a precalculus section of the book i'm solving. I am not supposed to use limits or derivatives.



I've already spent a couple of hours pondering on that, but that doesn't seem very obvious to me.




So now I'm wondering how could I find that limit in a precalculus manner?




Below is the graph in case someone is curious what it's really about:



polarplot









share|cite|improve this question










share|cite|improve this question




share|cite|improve this question









asked Aug 2 at 16:44









roman

3941312




3941312




put on hold as unclear what you're asking by Xander Henderson, Shailesh, Mostafa Ayaz, Lord Shark the Unknown, John B Aug 3 at 9:28


Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.






put on hold as unclear what you're asking by Xander Henderson, Shailesh, Mostafa Ayaz, Lord Shark the Unknown, John B Aug 3 at 9:28


Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.









  • 3




    You want to compute a limit without using limits? I am very confused about what it is that you are trying to accomplish...
    – Xander Henderson
    Aug 2 at 16:51






  • 1




    @XanderHenderson I need to know a value which is approached by $y$ as $phi to pi$, otherwise i can not build the plot.
    – roman
    Aug 2 at 16:57






  • 2




    Yes, but that involves computing a limit! Indeed, the answer that you have accepted requires computation of a limit. I don't understand what you are asking...
    – Xander Henderson
    Aug 2 at 17:01






  • 1




    @XanderHenderson I've accepted the below, because $sinx over x = 1$ can be shown geometrically without involving derivatives, and that's what i actually needed. I understand it's confusing to see "find a limit without a limit", but basically i intended to say "find a value of $y$" which is know as limit in calculus
    – roman
    Aug 2 at 17:10











  • It can be shown without derivatives, but it cannot be shown without limits, which is why I am confused.
    – Xander Henderson
    Aug 2 at 18:09












  • 3




    You want to compute a limit without using limits? I am very confused about what it is that you are trying to accomplish...
    – Xander Henderson
    Aug 2 at 16:51






  • 1




    @XanderHenderson I need to know a value which is approached by $y$ as $phi to pi$, otherwise i can not build the plot.
    – roman
    Aug 2 at 16:57






  • 2




    Yes, but that involves computing a limit! Indeed, the answer that you have accepted requires computation of a limit. I don't understand what you are asking...
    – Xander Henderson
    Aug 2 at 17:01






  • 1




    @XanderHenderson I've accepted the below, because $sinx over x = 1$ can be shown geometrically without involving derivatives, and that's what i actually needed. I understand it's confusing to see "find a limit without a limit", but basically i intended to say "find a value of $y$" which is know as limit in calculus
    – roman
    Aug 2 at 17:10











  • It can be shown without derivatives, but it cannot be shown without limits, which is why I am confused.
    – Xander Henderson
    Aug 2 at 18:09







3




3




You want to compute a limit without using limits? I am very confused about what it is that you are trying to accomplish...
– Xander Henderson
Aug 2 at 16:51




You want to compute a limit without using limits? I am very confused about what it is that you are trying to accomplish...
– Xander Henderson
Aug 2 at 16:51




1




1




@XanderHenderson I need to know a value which is approached by $y$ as $phi to pi$, otherwise i can not build the plot.
– roman
Aug 2 at 16:57




@XanderHenderson I need to know a value which is approached by $y$ as $phi to pi$, otherwise i can not build the plot.
– roman
Aug 2 at 16:57




2




2




Yes, but that involves computing a limit! Indeed, the answer that you have accepted requires computation of a limit. I don't understand what you are asking...
– Xander Henderson
Aug 2 at 17:01




Yes, but that involves computing a limit! Indeed, the answer that you have accepted requires computation of a limit. I don't understand what you are asking...
– Xander Henderson
Aug 2 at 17:01




1




1




@XanderHenderson I've accepted the below, because $sinx over x = 1$ can be shown geometrically without involving derivatives, and that's what i actually needed. I understand it's confusing to see "find a limit without a limit", but basically i intended to say "find a value of $y$" which is know as limit in calculus
– roman
Aug 2 at 17:10





@XanderHenderson I've accepted the below, because $sinx over x = 1$ can be shown geometrically without involving derivatives, and that's what i actually needed. I understand it's confusing to see "find a limit without a limit", but basically i intended to say "find a value of $y$" which is know as limit in calculus
– roman
Aug 2 at 17:10













It can be shown without derivatives, but it cannot be shown without limits, which is why I am confused.
– Xander Henderson
Aug 2 at 18:09




It can be shown without derivatives, but it cannot be shown without limits, which is why I am confused.
– Xander Henderson
Aug 2 at 18:09










2 Answers
2






active

oldest

votes

















up vote
2
down vote



accepted










We can make use of the well-known fact that $lim_x to 0 fracsin xx = 1$ and how it is proven. The main step in the prove is to note that $cos x le fracsin xx le 1$. We will use this idea.



Now note that $sin( phi - pi) = - sin phi $ and so we have:



$$ - phi le -fracphiphi-pi sin(phi -pi) le - phi cos(phi - pi)$$



Now you can see that the middle is bounded by $-pi$ from both sides as we get closer and closer to $pi$. Hence we can conclude that $y=-pi$, as $phi to pi$






share|cite|improve this answer

















  • 1




    ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
    – DonAntonio
    Aug 2 at 17:02

















up vote
1
down vote













The problem you describe is the very the definition of a limit.



$y(pi)$ is not defined, but $y$ is defined in the neighborhood of $pi$ and $y(phi)$ is close to $pi$ when $phi$ is close to $pi.$ That is a limit!



As this is pre-calc, and you don't have a formal definition of a limit, you are going to have to hand-waive to some degree. But, you don't need L'Hopitals rule.



You can evaluate $frac sin (phi)pi - phi$ via the squeeze theorem.



Probably easier to start with $frac sin xx$



$|sin x| le |x| le |tan x|$



And $|sin (pi - phi)| le |pi - phi| le |tan (pi - phi)|$



Geometrically:



enter image description here



The area of the large triangle is $frac 12 tan (pi - phi)$
The area of the section of the circle is $frac 12 (pi - phi)$
And the area of the smaller traingle is $frac 12 sin (pi - phi)$



And with a little algebra:



$cos (pi - phi)| le frac sin(pi - phi)pi - phi le 1$



and $phi cos (pi - phi)| le frac phipi - phisin (pi-phi) le phi$



Establishing an upper and lower bound, each close to $pi$ as $phi$ is close to $pi$






share|cite|improve this answer




























    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes








    up vote
    2
    down vote



    accepted










    We can make use of the well-known fact that $lim_x to 0 fracsin xx = 1$ and how it is proven. The main step in the prove is to note that $cos x le fracsin xx le 1$. We will use this idea.



    Now note that $sin( phi - pi) = - sin phi $ and so we have:



    $$ - phi le -fracphiphi-pi sin(phi -pi) le - phi cos(phi - pi)$$



    Now you can see that the middle is bounded by $-pi$ from both sides as we get closer and closer to $pi$. Hence we can conclude that $y=-pi$, as $phi to pi$






    share|cite|improve this answer

















    • 1




      ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
      – DonAntonio
      Aug 2 at 17:02














    up vote
    2
    down vote



    accepted










    We can make use of the well-known fact that $lim_x to 0 fracsin xx = 1$ and how it is proven. The main step in the prove is to note that $cos x le fracsin xx le 1$. We will use this idea.



    Now note that $sin( phi - pi) = - sin phi $ and so we have:



    $$ - phi le -fracphiphi-pi sin(phi -pi) le - phi cos(phi - pi)$$



    Now you can see that the middle is bounded by $-pi$ from both sides as we get closer and closer to $pi$. Hence we can conclude that $y=-pi$, as $phi to pi$






    share|cite|improve this answer

















    • 1




      ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
      – DonAntonio
      Aug 2 at 17:02












    up vote
    2
    down vote



    accepted







    up vote
    2
    down vote



    accepted






    We can make use of the well-known fact that $lim_x to 0 fracsin xx = 1$ and how it is proven. The main step in the prove is to note that $cos x le fracsin xx le 1$. We will use this idea.



    Now note that $sin( phi - pi) = - sin phi $ and so we have:



    $$ - phi le -fracphiphi-pi sin(phi -pi) le - phi cos(phi - pi)$$



    Now you can see that the middle is bounded by $-pi$ from both sides as we get closer and closer to $pi$. Hence we can conclude that $y=-pi$, as $phi to pi$






    share|cite|improve this answer













    We can make use of the well-known fact that $lim_x to 0 fracsin xx = 1$ and how it is proven. The main step in the prove is to note that $cos x le fracsin xx le 1$. We will use this idea.



    Now note that $sin( phi - pi) = - sin phi $ and so we have:



    $$ - phi le -fracphiphi-pi sin(phi -pi) le - phi cos(phi - pi)$$



    Now you can see that the middle is bounded by $-pi$ from both sides as we get closer and closer to $pi$. Hence we can conclude that $y=-pi$, as $phi to pi$







    share|cite|improve this answer













    share|cite|improve this answer



    share|cite|improve this answer











    answered Aug 2 at 16:56









    Stefan4024

    27.7k52974




    27.7k52974







    • 1




      ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
      – DonAntonio
      Aug 2 at 17:02












    • 1




      ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
      – DonAntonio
      Aug 2 at 17:02







    1




    1




    ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
    – DonAntonio
    Aug 2 at 17:02




    ..and this is just using limits, of course. Just as the comment by Xander said. Very weird question...
    – DonAntonio
    Aug 2 at 17:02










    up vote
    1
    down vote













    The problem you describe is the very the definition of a limit.



    $y(pi)$ is not defined, but $y$ is defined in the neighborhood of $pi$ and $y(phi)$ is close to $pi$ when $phi$ is close to $pi.$ That is a limit!



    As this is pre-calc, and you don't have a formal definition of a limit, you are going to have to hand-waive to some degree. But, you don't need L'Hopitals rule.



    You can evaluate $frac sin (phi)pi - phi$ via the squeeze theorem.



    Probably easier to start with $frac sin xx$



    $|sin x| le |x| le |tan x|$



    And $|sin (pi - phi)| le |pi - phi| le |tan (pi - phi)|$



    Geometrically:



    enter image description here



    The area of the large triangle is $frac 12 tan (pi - phi)$
    The area of the section of the circle is $frac 12 (pi - phi)$
    And the area of the smaller traingle is $frac 12 sin (pi - phi)$



    And with a little algebra:



    $cos (pi - phi)| le frac sin(pi - phi)pi - phi le 1$



    and $phi cos (pi - phi)| le frac phipi - phisin (pi-phi) le phi$



    Establishing an upper and lower bound, each close to $pi$ as $phi$ is close to $pi$






    share|cite|improve this answer

























      up vote
      1
      down vote













      The problem you describe is the very the definition of a limit.



      $y(pi)$ is not defined, but $y$ is defined in the neighborhood of $pi$ and $y(phi)$ is close to $pi$ when $phi$ is close to $pi.$ That is a limit!



      As this is pre-calc, and you don't have a formal definition of a limit, you are going to have to hand-waive to some degree. But, you don't need L'Hopitals rule.



      You can evaluate $frac sin (phi)pi - phi$ via the squeeze theorem.



      Probably easier to start with $frac sin xx$



      $|sin x| le |x| le |tan x|$



      And $|sin (pi - phi)| le |pi - phi| le |tan (pi - phi)|$



      Geometrically:



      enter image description here



      The area of the large triangle is $frac 12 tan (pi - phi)$
      The area of the section of the circle is $frac 12 (pi - phi)$
      And the area of the smaller traingle is $frac 12 sin (pi - phi)$



      And with a little algebra:



      $cos (pi - phi)| le frac sin(pi - phi)pi - phi le 1$



      and $phi cos (pi - phi)| le frac phipi - phisin (pi-phi) le phi$



      Establishing an upper and lower bound, each close to $pi$ as $phi$ is close to $pi$






      share|cite|improve this answer























        up vote
        1
        down vote










        up vote
        1
        down vote









        The problem you describe is the very the definition of a limit.



        $y(pi)$ is not defined, but $y$ is defined in the neighborhood of $pi$ and $y(phi)$ is close to $pi$ when $phi$ is close to $pi.$ That is a limit!



        As this is pre-calc, and you don't have a formal definition of a limit, you are going to have to hand-waive to some degree. But, you don't need L'Hopitals rule.



        You can evaluate $frac sin (phi)pi - phi$ via the squeeze theorem.



        Probably easier to start with $frac sin xx$



        $|sin x| le |x| le |tan x|$



        And $|sin (pi - phi)| le |pi - phi| le |tan (pi - phi)|$



        Geometrically:



        enter image description here



        The area of the large triangle is $frac 12 tan (pi - phi)$
        The area of the section of the circle is $frac 12 (pi - phi)$
        And the area of the smaller traingle is $frac 12 sin (pi - phi)$



        And with a little algebra:



        $cos (pi - phi)| le frac sin(pi - phi)pi - phi le 1$



        and $phi cos (pi - phi)| le frac phipi - phisin (pi-phi) le phi$



        Establishing an upper and lower bound, each close to $pi$ as $phi$ is close to $pi$






        share|cite|improve this answer













        The problem you describe is the very the definition of a limit.



        $y(pi)$ is not defined, but $y$ is defined in the neighborhood of $pi$ and $y(phi)$ is close to $pi$ when $phi$ is close to $pi.$ That is a limit!



        As this is pre-calc, and you don't have a formal definition of a limit, you are going to have to hand-waive to some degree. But, you don't need L'Hopitals rule.



        You can evaluate $frac sin (phi)pi - phi$ via the squeeze theorem.



        Probably easier to start with $frac sin xx$



        $|sin x| le |x| le |tan x|$



        And $|sin (pi - phi)| le |pi - phi| le |tan (pi - phi)|$



        Geometrically:



        enter image description here



        The area of the large triangle is $frac 12 tan (pi - phi)$
        The area of the section of the circle is $frac 12 (pi - phi)$
        And the area of the smaller traingle is $frac 12 sin (pi - phi)$



        And with a little algebra:



        $cos (pi - phi)| le frac sin(pi - phi)pi - phi le 1$



        and $phi cos (pi - phi)| le frac phipi - phisin (pi-phi) le phi$



        Establishing an upper and lower bound, each close to $pi$ as $phi$ is close to $pi$







        share|cite|improve this answer













        share|cite|improve this answer



        share|cite|improve this answer











        answered Aug 2 at 17:19









        Doug M

        39k31749




        39k31749












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