Radius of largest circle in an ellipse
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Consider an ellipse whose major and minor axis of length $10$ and $8$ unit respectively. Then the radius of the largest circle which can be inscribed in such an ellipse if the circle's center is one focus of the ellipse.
What I tried:
Assuming that major axis and minor axis of an ellipse along coordinate axis.($x$ and $y$ axis respectively)
Then equation of ellipse is $displaystyle frac{x^2}{a^2}+frac{y^2}{b^2}=1.$ where $a=5,b=4$
Then $displaystyle b^2=a^2(1-e^2)$ . getting $displaystyle e=3/5.$
coordinate of focus is $(pm ae,0)=(pm 3,0)$
Let equation of circle is $(xpm 3)^2+y^2=r^2$
How do I solve it from here?
conic-sections
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add a comment |
$begingroup$
Consider an ellipse whose major and minor axis of length $10$ and $8$ unit respectively. Then the radius of the largest circle which can be inscribed in such an ellipse if the circle's center is one focus of the ellipse.
What I tried:
Assuming that major axis and minor axis of an ellipse along coordinate axis.($x$ and $y$ axis respectively)
Then equation of ellipse is $displaystyle frac{x^2}{a^2}+frac{y^2}{b^2}=1.$ where $a=5,b=4$
Then $displaystyle b^2=a^2(1-e^2)$ . getting $displaystyle e=3/5.$
coordinate of focus is $(pm ae,0)=(pm 3,0)$
Let equation of circle is $(xpm 3)^2+y^2=r^2$
How do I solve it from here?
conic-sections
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$begingroup$
The nearer vertex is the closest point of the ellipse to the focus, so...
$endgroup$
– amd
Dec 29 '18 at 8:57
add a comment |
$begingroup$
Consider an ellipse whose major and minor axis of length $10$ and $8$ unit respectively. Then the radius of the largest circle which can be inscribed in such an ellipse if the circle's center is one focus of the ellipse.
What I tried:
Assuming that major axis and minor axis of an ellipse along coordinate axis.($x$ and $y$ axis respectively)
Then equation of ellipse is $displaystyle frac{x^2}{a^2}+frac{y^2}{b^2}=1.$ where $a=5,b=4$
Then $displaystyle b^2=a^2(1-e^2)$ . getting $displaystyle e=3/5.$
coordinate of focus is $(pm ae,0)=(pm 3,0)$
Let equation of circle is $(xpm 3)^2+y^2=r^2$
How do I solve it from here?
conic-sections
$endgroup$
Consider an ellipse whose major and minor axis of length $10$ and $8$ unit respectively. Then the radius of the largest circle which can be inscribed in such an ellipse if the circle's center is one focus of the ellipse.
What I tried:
Assuming that major axis and minor axis of an ellipse along coordinate axis.($x$ and $y$ axis respectively)
Then equation of ellipse is $displaystyle frac{x^2}{a^2}+frac{y^2}{b^2}=1.$ where $a=5,b=4$
Then $displaystyle b^2=a^2(1-e^2)$ . getting $displaystyle e=3/5.$
coordinate of focus is $(pm ae,0)=(pm 3,0)$
Let equation of circle is $(xpm 3)^2+y^2=r^2$
How do I solve it from here?
conic-sections
conic-sections
edited Dec 29 '18 at 5:11
David G. Stork
11.1k41432
11.1k41432
asked Dec 29 '18 at 4:55
jackyjacky
1,235815
1,235815
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The nearer vertex is the closest point of the ellipse to the focus, so...
$endgroup$
– amd
Dec 29 '18 at 8:57
add a comment |
$begingroup$
The nearer vertex is the closest point of the ellipse to the focus, so...
$endgroup$
– amd
Dec 29 '18 at 8:57
$begingroup$
The nearer vertex is the closest point of the ellipse to the focus, so...
$endgroup$
– amd
Dec 29 '18 at 8:57
$begingroup$
The nearer vertex is the closest point of the ellipse to the focus, so...
$endgroup$
– amd
Dec 29 '18 at 8:57
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
Denote $F, F'$ the foci of the ellipse. Assume we want to find the greatest circle inscribed in the ellipse, with center $F.$
The circle and the ellipse have a common tangent line and also a common normal in their point (points) of tangency $P,$ eventually also $P'$ if they are two.
The normal of an ellipse bisects the angle between the lines to the foci. In our case, the normal at $P$ bisects the angle between $PF$ and $PF'.$
The normal of a circle passes through its center.
From the above properties follows that $P,F,F'$ are collinear. Therefore, $P$ is the vertex of the ellipse lying at the major axis nearest to $F,$ and the equation of the circle is
$$(x-3)^2+y^2=|PF|^2=4$$
$endgroup$
add a comment |
$begingroup$
You want to minimize $r^2=(x-3)^2+y^2$ subject to $frac{x^2}{25}+frac{y^2}{16}=1$:
$$L(x,y,lambda)=(x-3)^2+y^2+lambdaleft(1-frac{x^2}{25}-frac{y^2}{16}right)\
begin{cases}L_x=2x-6-frac{2xlambda}{25}=0\ L_y=2y-frac{ylambda}{8}=0\ L_{lambda}=1-frac{x^2}{25}-frac{y^2}{16}=0end{cases} Rightarrow (x,y)=(pm5,0);\
r^2(5,0)=2^2 text{(min)}$$
Notes:
1) WolframAlpha answer.
2) If you minimize $r^2=(x+3)^2+y^2$ s.t. $frac{x^2}{25}+frac{y^2}{16}=1$, you will get $r^2(-5,0)=2^2$.
3) Foci $(pm c,0)$ can be found from $c^2=a^2-b^2=5^2-4^2=3^2 Rightarrow c=pm 3$.
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add a comment |
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Eliminate $y$. Then find the value of $x$ where $r$ is minimised.
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add a comment |
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Try to obtain a $r$ such that the circle intersects with the ellipse at exactly 2 points. This is when the circle is tangent to the ellipse.
$endgroup$
2
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There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
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@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
add a comment |
Your Answer
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4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Denote $F, F'$ the foci of the ellipse. Assume we want to find the greatest circle inscribed in the ellipse, with center $F.$
The circle and the ellipse have a common tangent line and also a common normal in their point (points) of tangency $P,$ eventually also $P'$ if they are two.
The normal of an ellipse bisects the angle between the lines to the foci. In our case, the normal at $P$ bisects the angle between $PF$ and $PF'.$
The normal of a circle passes through its center.
From the above properties follows that $P,F,F'$ are collinear. Therefore, $P$ is the vertex of the ellipse lying at the major axis nearest to $F,$ and the equation of the circle is
$$(x-3)^2+y^2=|PF|^2=4$$
$endgroup$
add a comment |
$begingroup$
Denote $F, F'$ the foci of the ellipse. Assume we want to find the greatest circle inscribed in the ellipse, with center $F.$
The circle and the ellipse have a common tangent line and also a common normal in their point (points) of tangency $P,$ eventually also $P'$ if they are two.
The normal of an ellipse bisects the angle between the lines to the foci. In our case, the normal at $P$ bisects the angle between $PF$ and $PF'.$
The normal of a circle passes through its center.
From the above properties follows that $P,F,F'$ are collinear. Therefore, $P$ is the vertex of the ellipse lying at the major axis nearest to $F,$ and the equation of the circle is
$$(x-3)^2+y^2=|PF|^2=4$$
$endgroup$
add a comment |
$begingroup$
Denote $F, F'$ the foci of the ellipse. Assume we want to find the greatest circle inscribed in the ellipse, with center $F.$
The circle and the ellipse have a common tangent line and also a common normal in their point (points) of tangency $P,$ eventually also $P'$ if they are two.
The normal of an ellipse bisects the angle between the lines to the foci. In our case, the normal at $P$ bisects the angle between $PF$ and $PF'.$
The normal of a circle passes through its center.
From the above properties follows that $P,F,F'$ are collinear. Therefore, $P$ is the vertex of the ellipse lying at the major axis nearest to $F,$ and the equation of the circle is
$$(x-3)^2+y^2=|PF|^2=4$$
$endgroup$
Denote $F, F'$ the foci of the ellipse. Assume we want to find the greatest circle inscribed in the ellipse, with center $F.$
The circle and the ellipse have a common tangent line and also a common normal in their point (points) of tangency $P,$ eventually also $P'$ if they are two.
The normal of an ellipse bisects the angle between the lines to the foci. In our case, the normal at $P$ bisects the angle between $PF$ and $PF'.$
The normal of a circle passes through its center.
From the above properties follows that $P,F,F'$ are collinear. Therefore, $P$ is the vertex of the ellipse lying at the major axis nearest to $F,$ and the equation of the circle is
$$(x-3)^2+y^2=|PF|^2=4$$
answered Dec 29 '18 at 22:29
user376343user376343
3,9584829
3,9584829
add a comment |
add a comment |
$begingroup$
You want to minimize $r^2=(x-3)^2+y^2$ subject to $frac{x^2}{25}+frac{y^2}{16}=1$:
$$L(x,y,lambda)=(x-3)^2+y^2+lambdaleft(1-frac{x^2}{25}-frac{y^2}{16}right)\
begin{cases}L_x=2x-6-frac{2xlambda}{25}=0\ L_y=2y-frac{ylambda}{8}=0\ L_{lambda}=1-frac{x^2}{25}-frac{y^2}{16}=0end{cases} Rightarrow (x,y)=(pm5,0);\
r^2(5,0)=2^2 text{(min)}$$
Notes:
1) WolframAlpha answer.
2) If you minimize $r^2=(x+3)^2+y^2$ s.t. $frac{x^2}{25}+frac{y^2}{16}=1$, you will get $r^2(-5,0)=2^2$.
3) Foci $(pm c,0)$ can be found from $c^2=a^2-b^2=5^2-4^2=3^2 Rightarrow c=pm 3$.
$endgroup$
add a comment |
$begingroup$
You want to minimize $r^2=(x-3)^2+y^2$ subject to $frac{x^2}{25}+frac{y^2}{16}=1$:
$$L(x,y,lambda)=(x-3)^2+y^2+lambdaleft(1-frac{x^2}{25}-frac{y^2}{16}right)\
begin{cases}L_x=2x-6-frac{2xlambda}{25}=0\ L_y=2y-frac{ylambda}{8}=0\ L_{lambda}=1-frac{x^2}{25}-frac{y^2}{16}=0end{cases} Rightarrow (x,y)=(pm5,0);\
r^2(5,0)=2^2 text{(min)}$$
Notes:
1) WolframAlpha answer.
2) If you minimize $r^2=(x+3)^2+y^2$ s.t. $frac{x^2}{25}+frac{y^2}{16}=1$, you will get $r^2(-5,0)=2^2$.
3) Foci $(pm c,0)$ can be found from $c^2=a^2-b^2=5^2-4^2=3^2 Rightarrow c=pm 3$.
$endgroup$
add a comment |
$begingroup$
You want to minimize $r^2=(x-3)^2+y^2$ subject to $frac{x^2}{25}+frac{y^2}{16}=1$:
$$L(x,y,lambda)=(x-3)^2+y^2+lambdaleft(1-frac{x^2}{25}-frac{y^2}{16}right)\
begin{cases}L_x=2x-6-frac{2xlambda}{25}=0\ L_y=2y-frac{ylambda}{8}=0\ L_{lambda}=1-frac{x^2}{25}-frac{y^2}{16}=0end{cases} Rightarrow (x,y)=(pm5,0);\
r^2(5,0)=2^2 text{(min)}$$
Notes:
1) WolframAlpha answer.
2) If you minimize $r^2=(x+3)^2+y^2$ s.t. $frac{x^2}{25}+frac{y^2}{16}=1$, you will get $r^2(-5,0)=2^2$.
3) Foci $(pm c,0)$ can be found from $c^2=a^2-b^2=5^2-4^2=3^2 Rightarrow c=pm 3$.
$endgroup$
You want to minimize $r^2=(x-3)^2+y^2$ subject to $frac{x^2}{25}+frac{y^2}{16}=1$:
$$L(x,y,lambda)=(x-3)^2+y^2+lambdaleft(1-frac{x^2}{25}-frac{y^2}{16}right)\
begin{cases}L_x=2x-6-frac{2xlambda}{25}=0\ L_y=2y-frac{ylambda}{8}=0\ L_{lambda}=1-frac{x^2}{25}-frac{y^2}{16}=0end{cases} Rightarrow (x,y)=(pm5,0);\
r^2(5,0)=2^2 text{(min)}$$
Notes:
1) WolframAlpha answer.
2) If you minimize $r^2=(x+3)^2+y^2$ s.t. $frac{x^2}{25}+frac{y^2}{16}=1$, you will get $r^2(-5,0)=2^2$.
3) Foci $(pm c,0)$ can be found from $c^2=a^2-b^2=5^2-4^2=3^2 Rightarrow c=pm 3$.
answered Dec 29 '18 at 7:02
farruhotafarruhota
21.3k2841
21.3k2841
add a comment |
add a comment |
$begingroup$
Eliminate $y$. Then find the value of $x$ where $r$ is minimised.
$endgroup$
add a comment |
$begingroup$
Eliminate $y$. Then find the value of $x$ where $r$ is minimised.
$endgroup$
add a comment |
$begingroup$
Eliminate $y$. Then find the value of $x$ where $r$ is minimised.
$endgroup$
Eliminate $y$. Then find the value of $x$ where $r$ is minimised.
answered Dec 29 '18 at 5:47
Empy2Empy2
33.6k12462
33.6k12462
add a comment |
add a comment |
$begingroup$
Try to obtain a $r$ such that the circle intersects with the ellipse at exactly 2 points. This is when the circle is tangent to the ellipse.
$endgroup$
2
$begingroup$
There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
$begingroup$
@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
add a comment |
$begingroup$
Try to obtain a $r$ such that the circle intersects with the ellipse at exactly 2 points. This is when the circle is tangent to the ellipse.
$endgroup$
2
$begingroup$
There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
$begingroup$
@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
add a comment |
$begingroup$
Try to obtain a $r$ such that the circle intersects with the ellipse at exactly 2 points. This is when the circle is tangent to the ellipse.
$endgroup$
Try to obtain a $r$ such that the circle intersects with the ellipse at exactly 2 points. This is when the circle is tangent to the ellipse.
answered Dec 29 '18 at 5:16
TreborTrebor
92815
92815
2
$begingroup$
There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
$begingroup$
@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
add a comment |
2
$begingroup$
There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
$begingroup$
@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
2
2
$begingroup$
There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
$begingroup$
There is an infinite number of circles centered at a focus that intersect the ellipse at exactly two points. That condition is insufficient for tangency. In fact, there are exactly 2 tangent circles and their intersections with the ellipse each consist of a single point.
$endgroup$
– amd
Dec 29 '18 at 9:00
$begingroup$
@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
$begingroup$
@Trebor, if the center of the circle is in a focus of the ellipse, there are never two points of contact. This is one of properties of ellipse. If there are two common points, the circle cuts the ellipse.
$endgroup$
– user376343
Dec 30 '18 at 18:53
add a comment |
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$begingroup$
The nearer vertex is the closest point of the ellipse to the focus, so...
$endgroup$
– amd
Dec 29 '18 at 8:57