Relationship of radius of sphere to an inscribed right circular cylinder for max and min values












0












$begingroup$


I cannot seem to find the correlation between having an interval of a radius of a sphere with finding the greatest lateral surface area of a right circular cylinder inscribed in it.



The question goes like this:




Calculate the dimensions of the right circular cylinder of the greatest lateral surface area that can be inscribed in a sphere of radius $6$ inches.




Since the surface area of a cylinder is $2pi rh$ and since the distance of the sphere's center and the tip of the cylinder is equal to the radius ($6$ inches), then by Pythagorean theorem,



$$left(frac h 2right)^2 + r^2 =36$$



$frac h 2$ since the center of the sphere is also the midpoint of the cylinder's height.



Then I found $h$:



$$h= 2sqrt{36-r^2}$$



By this, we can have the equation $A(r)=SA$ of cylinder and



$$A(r)=2pi r cdot 2sqrt{36-r^2}$$



But then my question is what is the connection of this function with the interval $0le rle6$?



Thanks










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$endgroup$












  • $begingroup$
    Mathematical formulae look better in $LaTeX$. Here is a quick tutorial.
    $endgroup$
    – Τίμων
    Apr 9 '16 at 14:05










  • $begingroup$
    I don't exactly get the question, but if you mean where the radius $R$ of the sphere appears, then the answer is: as $R^2$ in the expression for $h = 2 sqrt{R^2-r^2}$.
    $endgroup$
    – Seven
    Apr 9 '16 at 18:38












  • $begingroup$
    I think i am about to grasp the answer to my question but what is the implication of the interval [0,6] to this problem?
    $endgroup$
    – user311699
    Apr 10 '16 at 3:15
















0












$begingroup$


I cannot seem to find the correlation between having an interval of a radius of a sphere with finding the greatest lateral surface area of a right circular cylinder inscribed in it.



The question goes like this:




Calculate the dimensions of the right circular cylinder of the greatest lateral surface area that can be inscribed in a sphere of radius $6$ inches.




Since the surface area of a cylinder is $2pi rh$ and since the distance of the sphere's center and the tip of the cylinder is equal to the radius ($6$ inches), then by Pythagorean theorem,



$$left(frac h 2right)^2 + r^2 =36$$



$frac h 2$ since the center of the sphere is also the midpoint of the cylinder's height.



Then I found $h$:



$$h= 2sqrt{36-r^2}$$



By this, we can have the equation $A(r)=SA$ of cylinder and



$$A(r)=2pi r cdot 2sqrt{36-r^2}$$



But then my question is what is the connection of this function with the interval $0le rle6$?



Thanks










share|cite|improve this question











$endgroup$












  • $begingroup$
    Mathematical formulae look better in $LaTeX$. Here is a quick tutorial.
    $endgroup$
    – Τίμων
    Apr 9 '16 at 14:05










  • $begingroup$
    I don't exactly get the question, but if you mean where the radius $R$ of the sphere appears, then the answer is: as $R^2$ in the expression for $h = 2 sqrt{R^2-r^2}$.
    $endgroup$
    – Seven
    Apr 9 '16 at 18:38












  • $begingroup$
    I think i am about to grasp the answer to my question but what is the implication of the interval [0,6] to this problem?
    $endgroup$
    – user311699
    Apr 10 '16 at 3:15














0












0








0





$begingroup$


I cannot seem to find the correlation between having an interval of a radius of a sphere with finding the greatest lateral surface area of a right circular cylinder inscribed in it.



The question goes like this:




Calculate the dimensions of the right circular cylinder of the greatest lateral surface area that can be inscribed in a sphere of radius $6$ inches.




Since the surface area of a cylinder is $2pi rh$ and since the distance of the sphere's center and the tip of the cylinder is equal to the radius ($6$ inches), then by Pythagorean theorem,



$$left(frac h 2right)^2 + r^2 =36$$



$frac h 2$ since the center of the sphere is also the midpoint of the cylinder's height.



Then I found $h$:



$$h= 2sqrt{36-r^2}$$



By this, we can have the equation $A(r)=SA$ of cylinder and



$$A(r)=2pi r cdot 2sqrt{36-r^2}$$



But then my question is what is the connection of this function with the interval $0le rle6$?



Thanks










share|cite|improve this question











$endgroup$




I cannot seem to find the correlation between having an interval of a radius of a sphere with finding the greatest lateral surface area of a right circular cylinder inscribed in it.



The question goes like this:




Calculate the dimensions of the right circular cylinder of the greatest lateral surface area that can be inscribed in a sphere of radius $6$ inches.




Since the surface area of a cylinder is $2pi rh$ and since the distance of the sphere's center and the tip of the cylinder is equal to the radius ($6$ inches), then by Pythagorean theorem,



$$left(frac h 2right)^2 + r^2 =36$$



$frac h 2$ since the center of the sphere is also the midpoint of the cylinder's height.



Then I found $h$:



$$h= 2sqrt{36-r^2}$$



By this, we can have the equation $A(r)=SA$ of cylinder and



$$A(r)=2pi r cdot 2sqrt{36-r^2}$$



But then my question is what is the connection of this function with the interval $0le rle6$?



Thanks







calculus derivatives optimization problem-solving






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share|cite|improve this question








edited Apr 9 '16 at 14:09









Τίμων

1,3382824




1,3382824










asked Apr 9 '16 at 14:02









user311699user311699

105




105












  • $begingroup$
    Mathematical formulae look better in $LaTeX$. Here is a quick tutorial.
    $endgroup$
    – Τίμων
    Apr 9 '16 at 14:05










  • $begingroup$
    I don't exactly get the question, but if you mean where the radius $R$ of the sphere appears, then the answer is: as $R^2$ in the expression for $h = 2 sqrt{R^2-r^2}$.
    $endgroup$
    – Seven
    Apr 9 '16 at 18:38












  • $begingroup$
    I think i am about to grasp the answer to my question but what is the implication of the interval [0,6] to this problem?
    $endgroup$
    – user311699
    Apr 10 '16 at 3:15


















  • $begingroup$
    Mathematical formulae look better in $LaTeX$. Here is a quick tutorial.
    $endgroup$
    – Τίμων
    Apr 9 '16 at 14:05










  • $begingroup$
    I don't exactly get the question, but if you mean where the radius $R$ of the sphere appears, then the answer is: as $R^2$ in the expression for $h = 2 sqrt{R^2-r^2}$.
    $endgroup$
    – Seven
    Apr 9 '16 at 18:38












  • $begingroup$
    I think i am about to grasp the answer to my question but what is the implication of the interval [0,6] to this problem?
    $endgroup$
    – user311699
    Apr 10 '16 at 3:15
















$begingroup$
Mathematical formulae look better in $LaTeX$. Here is a quick tutorial.
$endgroup$
– Τίμων
Apr 9 '16 at 14:05




$begingroup$
Mathematical formulae look better in $LaTeX$. Here is a quick tutorial.
$endgroup$
– Τίμων
Apr 9 '16 at 14:05












$begingroup$
I don't exactly get the question, but if you mean where the radius $R$ of the sphere appears, then the answer is: as $R^2$ in the expression for $h = 2 sqrt{R^2-r^2}$.
$endgroup$
– Seven
Apr 9 '16 at 18:38






$begingroup$
I don't exactly get the question, but if you mean where the radius $R$ of the sphere appears, then the answer is: as $R^2$ in the expression for $h = 2 sqrt{R^2-r^2}$.
$endgroup$
– Seven
Apr 9 '16 at 18:38














$begingroup$
I think i am about to grasp the answer to my question but what is the implication of the interval [0,6] to this problem?
$endgroup$
– user311699
Apr 10 '16 at 3:15




$begingroup$
I think i am about to grasp the answer to my question but what is the implication of the interval [0,6] to this problem?
$endgroup$
– user311699
Apr 10 '16 at 3:15










1 Answer
1






active

oldest

votes


















0












$begingroup$

You don't say so explicitly, but your formulas imply that the
symbol $r$ represents the radius of the cylinder and $h$ is the height
of the cylinder.



There are many differently-shaped cylinders that can fit inside a
given sphere. If the radius of the sphere is $6$,
near one extreme, you have a very skinny cylinder whose
height is very close to $12$ and whose radius is close to zero.
Near the other extreme, you have a cylinder that is almost a flat disk
with a very small "height" (or thickness) $h$ and a radius that is
barely less than $6$.



We can even take the cylinder all the way to the extreme where it is
only a degenerate cylinder: $h=12$, $r=0$ describes a line segment that
just exactly fits in the sphere, and $h=0$, $r=6$ describes a flat disk
that also exactly fits in the sphere.



There's not much point in considering $r < 0$, because what is a cylinder
with a negative radius? (Actually there is a way to interpret such a thing,
but if you allow, for example, a cylinder of radius $-2$, it's identical to
a cylinder of radius $2$; so you have found no new cylinders but you have
to rewrite formulas such as $A(r) = 2pi rh$ because the lateral area
of a cylinder is not less than zero. It's easier, and we don't miss
any answers, if we consider only cylinders such that $r geq 0$.)



There is certainly no point in considering a cylinder with radius $r > 6$
if the radius of your sphere is $6$, because there is no way such
a cylinder could possibly fit inside the sphere.



You could also come to these conclusions by examining the formula
you found,
$$A(r)=2pi r cdot 2sqrt{36-r^2},$$
because $r < 0$ gives a negative area (which can't be correct)
and $r > 6$ asks you to take the square root of a negative number,
which doesn't work.
But I think it's a good idea to have reasons based in the original
problem statement that say why a variable $r$ should have only a certain
possible set of values, because there's no guarantee that every
restriction that the problem statement requires will also be
enforced by whatever formulas we derive.



So we are OK with $r=0$ (if we accept a line segment as a degenerate cylinder of radius zero), we are OK with $r=6$ (if we accept a circle as a degenerate cylinder of height zero), and we are OK with
anything in between--just set $h= 2sqrt{36-r^2}$, as you discovered,
and the cylinder will fit perfectly.
But $r < 0$ is useless to consider and $r > 6$ is impossible.
So $0 leq r leq 6$ describes every cylinder that we could ever want
to inscribe in a sphere of radius $6$.






share|cite|improve this answer











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    1 Answer
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    $begingroup$

    You don't say so explicitly, but your formulas imply that the
    symbol $r$ represents the radius of the cylinder and $h$ is the height
    of the cylinder.



    There are many differently-shaped cylinders that can fit inside a
    given sphere. If the radius of the sphere is $6$,
    near one extreme, you have a very skinny cylinder whose
    height is very close to $12$ and whose radius is close to zero.
    Near the other extreme, you have a cylinder that is almost a flat disk
    with a very small "height" (or thickness) $h$ and a radius that is
    barely less than $6$.



    We can even take the cylinder all the way to the extreme where it is
    only a degenerate cylinder: $h=12$, $r=0$ describes a line segment that
    just exactly fits in the sphere, and $h=0$, $r=6$ describes a flat disk
    that also exactly fits in the sphere.



    There's not much point in considering $r < 0$, because what is a cylinder
    with a negative radius? (Actually there is a way to interpret such a thing,
    but if you allow, for example, a cylinder of radius $-2$, it's identical to
    a cylinder of radius $2$; so you have found no new cylinders but you have
    to rewrite formulas such as $A(r) = 2pi rh$ because the lateral area
    of a cylinder is not less than zero. It's easier, and we don't miss
    any answers, if we consider only cylinders such that $r geq 0$.)



    There is certainly no point in considering a cylinder with radius $r > 6$
    if the radius of your sphere is $6$, because there is no way such
    a cylinder could possibly fit inside the sphere.



    You could also come to these conclusions by examining the formula
    you found,
    $$A(r)=2pi r cdot 2sqrt{36-r^2},$$
    because $r < 0$ gives a negative area (which can't be correct)
    and $r > 6$ asks you to take the square root of a negative number,
    which doesn't work.
    But I think it's a good idea to have reasons based in the original
    problem statement that say why a variable $r$ should have only a certain
    possible set of values, because there's no guarantee that every
    restriction that the problem statement requires will also be
    enforced by whatever formulas we derive.



    So we are OK with $r=0$ (if we accept a line segment as a degenerate cylinder of radius zero), we are OK with $r=6$ (if we accept a circle as a degenerate cylinder of height zero), and we are OK with
    anything in between--just set $h= 2sqrt{36-r^2}$, as you discovered,
    and the cylinder will fit perfectly.
    But $r < 0$ is useless to consider and $r > 6$ is impossible.
    So $0 leq r leq 6$ describes every cylinder that we could ever want
    to inscribe in a sphere of radius $6$.






    share|cite|improve this answer











    $endgroup$


















      0












      $begingroup$

      You don't say so explicitly, but your formulas imply that the
      symbol $r$ represents the radius of the cylinder and $h$ is the height
      of the cylinder.



      There are many differently-shaped cylinders that can fit inside a
      given sphere. If the radius of the sphere is $6$,
      near one extreme, you have a very skinny cylinder whose
      height is very close to $12$ and whose radius is close to zero.
      Near the other extreme, you have a cylinder that is almost a flat disk
      with a very small "height" (or thickness) $h$ and a radius that is
      barely less than $6$.



      We can even take the cylinder all the way to the extreme where it is
      only a degenerate cylinder: $h=12$, $r=0$ describes a line segment that
      just exactly fits in the sphere, and $h=0$, $r=6$ describes a flat disk
      that also exactly fits in the sphere.



      There's not much point in considering $r < 0$, because what is a cylinder
      with a negative radius? (Actually there is a way to interpret such a thing,
      but if you allow, for example, a cylinder of radius $-2$, it's identical to
      a cylinder of radius $2$; so you have found no new cylinders but you have
      to rewrite formulas such as $A(r) = 2pi rh$ because the lateral area
      of a cylinder is not less than zero. It's easier, and we don't miss
      any answers, if we consider only cylinders such that $r geq 0$.)



      There is certainly no point in considering a cylinder with radius $r > 6$
      if the radius of your sphere is $6$, because there is no way such
      a cylinder could possibly fit inside the sphere.



      You could also come to these conclusions by examining the formula
      you found,
      $$A(r)=2pi r cdot 2sqrt{36-r^2},$$
      because $r < 0$ gives a negative area (which can't be correct)
      and $r > 6$ asks you to take the square root of a negative number,
      which doesn't work.
      But I think it's a good idea to have reasons based in the original
      problem statement that say why a variable $r$ should have only a certain
      possible set of values, because there's no guarantee that every
      restriction that the problem statement requires will also be
      enforced by whatever formulas we derive.



      So we are OK with $r=0$ (if we accept a line segment as a degenerate cylinder of radius zero), we are OK with $r=6$ (if we accept a circle as a degenerate cylinder of height zero), and we are OK with
      anything in between--just set $h= 2sqrt{36-r^2}$, as you discovered,
      and the cylinder will fit perfectly.
      But $r < 0$ is useless to consider and $r > 6$ is impossible.
      So $0 leq r leq 6$ describes every cylinder that we could ever want
      to inscribe in a sphere of radius $6$.






      share|cite|improve this answer











      $endgroup$
















        0












        0








        0





        $begingroup$

        You don't say so explicitly, but your formulas imply that the
        symbol $r$ represents the radius of the cylinder and $h$ is the height
        of the cylinder.



        There are many differently-shaped cylinders that can fit inside a
        given sphere. If the radius of the sphere is $6$,
        near one extreme, you have a very skinny cylinder whose
        height is very close to $12$ and whose radius is close to zero.
        Near the other extreme, you have a cylinder that is almost a flat disk
        with a very small "height" (or thickness) $h$ and a radius that is
        barely less than $6$.



        We can even take the cylinder all the way to the extreme where it is
        only a degenerate cylinder: $h=12$, $r=0$ describes a line segment that
        just exactly fits in the sphere, and $h=0$, $r=6$ describes a flat disk
        that also exactly fits in the sphere.



        There's not much point in considering $r < 0$, because what is a cylinder
        with a negative radius? (Actually there is a way to interpret such a thing,
        but if you allow, for example, a cylinder of radius $-2$, it's identical to
        a cylinder of radius $2$; so you have found no new cylinders but you have
        to rewrite formulas such as $A(r) = 2pi rh$ because the lateral area
        of a cylinder is not less than zero. It's easier, and we don't miss
        any answers, if we consider only cylinders such that $r geq 0$.)



        There is certainly no point in considering a cylinder with radius $r > 6$
        if the radius of your sphere is $6$, because there is no way such
        a cylinder could possibly fit inside the sphere.



        You could also come to these conclusions by examining the formula
        you found,
        $$A(r)=2pi r cdot 2sqrt{36-r^2},$$
        because $r < 0$ gives a negative area (which can't be correct)
        and $r > 6$ asks you to take the square root of a negative number,
        which doesn't work.
        But I think it's a good idea to have reasons based in the original
        problem statement that say why a variable $r$ should have only a certain
        possible set of values, because there's no guarantee that every
        restriction that the problem statement requires will also be
        enforced by whatever formulas we derive.



        So we are OK with $r=0$ (if we accept a line segment as a degenerate cylinder of radius zero), we are OK with $r=6$ (if we accept a circle as a degenerate cylinder of height zero), and we are OK with
        anything in between--just set $h= 2sqrt{36-r^2}$, as you discovered,
        and the cylinder will fit perfectly.
        But $r < 0$ is useless to consider and $r > 6$ is impossible.
        So $0 leq r leq 6$ describes every cylinder that we could ever want
        to inscribe in a sphere of radius $6$.






        share|cite|improve this answer











        $endgroup$



        You don't say so explicitly, but your formulas imply that the
        symbol $r$ represents the radius of the cylinder and $h$ is the height
        of the cylinder.



        There are many differently-shaped cylinders that can fit inside a
        given sphere. If the radius of the sphere is $6$,
        near one extreme, you have a very skinny cylinder whose
        height is very close to $12$ and whose radius is close to zero.
        Near the other extreme, you have a cylinder that is almost a flat disk
        with a very small "height" (or thickness) $h$ and a radius that is
        barely less than $6$.



        We can even take the cylinder all the way to the extreme where it is
        only a degenerate cylinder: $h=12$, $r=0$ describes a line segment that
        just exactly fits in the sphere, and $h=0$, $r=6$ describes a flat disk
        that also exactly fits in the sphere.



        There's not much point in considering $r < 0$, because what is a cylinder
        with a negative radius? (Actually there is a way to interpret such a thing,
        but if you allow, for example, a cylinder of radius $-2$, it's identical to
        a cylinder of radius $2$; so you have found no new cylinders but you have
        to rewrite formulas such as $A(r) = 2pi rh$ because the lateral area
        of a cylinder is not less than zero. It's easier, and we don't miss
        any answers, if we consider only cylinders such that $r geq 0$.)



        There is certainly no point in considering a cylinder with radius $r > 6$
        if the radius of your sphere is $6$, because there is no way such
        a cylinder could possibly fit inside the sphere.



        You could also come to these conclusions by examining the formula
        you found,
        $$A(r)=2pi r cdot 2sqrt{36-r^2},$$
        because $r < 0$ gives a negative area (which can't be correct)
        and $r > 6$ asks you to take the square root of a negative number,
        which doesn't work.
        But I think it's a good idea to have reasons based in the original
        problem statement that say why a variable $r$ should have only a certain
        possible set of values, because there's no guarantee that every
        restriction that the problem statement requires will also be
        enforced by whatever formulas we derive.



        So we are OK with $r=0$ (if we accept a line segment as a degenerate cylinder of radius zero), we are OK with $r=6$ (if we accept a circle as a degenerate cylinder of height zero), and we are OK with
        anything in between--just set $h= 2sqrt{36-r^2}$, as you discovered,
        and the cylinder will fit perfectly.
        But $r < 0$ is useless to consider and $r > 6$ is impossible.
        So $0 leq r leq 6$ describes every cylinder that we could ever want
        to inscribe in a sphere of radius $6$.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Aug 25 '18 at 15:54

























        answered Apr 26 '16 at 2:51









        David KDavid K

        54.5k343120




        54.5k343120






























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