Finding equation for conic section given five points
$begingroup$
Problem:
Given the points $$(0,1),(0,-1),(2,0),(-2,0),(1,1)$$ find the equation for the conic section that passes through these points.
My attempt:
Using the general equation for a conic section, $$ax^2+bxy+cy^2+dx+ey+f=0$$ I inserted the points to make a system of 5 equations, which was reduced to the following, with $a$ as a free variable.
$$b=-a \c=4a \d=e=0 \f=-4a$$
Putting this into the general equation for a conic, I got $$ax^2-axy+4ay^2-4a=0 (1)$$
My question:
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of $b^2+4ac$, where I ended up with $b^2-4ac = -15a^2 < 0$ which is supposed to be an ellipse. However, I can't find my way back to the place where I read this. Looking at the points on a graph, it seems visually obvious that this is an ellipse. However, I can't prove it unless the mentioned method is valid.
If it really is an ellipse, how can I algebraically manipulate $(1)$ into the form most known for ellipses?
conic-sections
$endgroup$
add a comment |
$begingroup$
Problem:
Given the points $$(0,1),(0,-1),(2,0),(-2,0),(1,1)$$ find the equation for the conic section that passes through these points.
My attempt:
Using the general equation for a conic section, $$ax^2+bxy+cy^2+dx+ey+f=0$$ I inserted the points to make a system of 5 equations, which was reduced to the following, with $a$ as a free variable.
$$b=-a \c=4a \d=e=0 \f=-4a$$
Putting this into the general equation for a conic, I got $$ax^2-axy+4ay^2-4a=0 (1)$$
My question:
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of $b^2+4ac$, where I ended up with $b^2-4ac = -15a^2 < 0$ which is supposed to be an ellipse. However, I can't find my way back to the place where I read this. Looking at the points on a graph, it seems visually obvious that this is an ellipse. However, I can't prove it unless the mentioned method is valid.
If it really is an ellipse, how can I algebraically manipulate $(1)$ into the form most known for ellipses?
conic-sections
$endgroup$
$begingroup$
The form most known for ellipses has major and minor axes parallel to the $x$ and $y$ axes, and thus no cross term (i.e. $b = 0$). Without rotating the coordinate system there is no hope of writing this as $frac{(x-i)^2}{j} + frac{(y-k)^2}{l} = 1$
$endgroup$
– Arthur
Jan 10 '15 at 1:52
$begingroup$
@Amzoti - Yes, there is the $b^2-4ac$ bit I was talking about. Using that calculator confirmed what I thought, but I still need to prove that $x^2-xy+4y^2-4=0$ represents an ellipse. Also, any non-zero $a$ will yield the same ellipse. This is obvious since you can factor it out from $(1)$.
$endgroup$
– Alec
Jan 10 '15 at 2:14
$begingroup$
@Arthur - You're right. Drawing the equation in Geogebra shows a slightly rotated ellipse. Could $(1)$ after factoring and striking $a neq 0$ be the answer to the problem?
$endgroup$
– Alec
Jan 10 '15 at 2:16
$begingroup$
Why not to use the method described in en.wikipedia.org/wiki/…
$endgroup$
– Claude Leibovici
Jan 10 '15 at 4:23
$begingroup$
For finding the equation in the first place, solving is the system requires a guess about which variable to leave "free"; and, of course, there's all that solving work to do. Using the determinant mentioned in this answer, one generates the equation directly ... albeit with the work of expanding a $6times 6$ determinant.
$endgroup$
– Blue
Aug 18 '18 at 16:04
add a comment |
$begingroup$
Problem:
Given the points $$(0,1),(0,-1),(2,0),(-2,0),(1,1)$$ find the equation for the conic section that passes through these points.
My attempt:
Using the general equation for a conic section, $$ax^2+bxy+cy^2+dx+ey+f=0$$ I inserted the points to make a system of 5 equations, which was reduced to the following, with $a$ as a free variable.
$$b=-a \c=4a \d=e=0 \f=-4a$$
Putting this into the general equation for a conic, I got $$ax^2-axy+4ay^2-4a=0 (1)$$
My question:
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of $b^2+4ac$, where I ended up with $b^2-4ac = -15a^2 < 0$ which is supposed to be an ellipse. However, I can't find my way back to the place where I read this. Looking at the points on a graph, it seems visually obvious that this is an ellipse. However, I can't prove it unless the mentioned method is valid.
If it really is an ellipse, how can I algebraically manipulate $(1)$ into the form most known for ellipses?
conic-sections
$endgroup$
Problem:
Given the points $$(0,1),(0,-1),(2,0),(-2,0),(1,1)$$ find the equation for the conic section that passes through these points.
My attempt:
Using the general equation for a conic section, $$ax^2+bxy+cy^2+dx+ey+f=0$$ I inserted the points to make a system of 5 equations, which was reduced to the following, with $a$ as a free variable.
$$b=-a \c=4a \d=e=0 \f=-4a$$
Putting this into the general equation for a conic, I got $$ax^2-axy+4ay^2-4a=0 (1)$$
My question:
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of $b^2+4ac$, where I ended up with $b^2-4ac = -15a^2 < 0$ which is supposed to be an ellipse. However, I can't find my way back to the place where I read this. Looking at the points on a graph, it seems visually obvious that this is an ellipse. However, I can't prove it unless the mentioned method is valid.
If it really is an ellipse, how can I algebraically manipulate $(1)$ into the form most known for ellipses?
conic-sections
conic-sections
asked Jan 10 '15 at 1:37
AlecAlec
2,20211539
2,20211539
$begingroup$
The form most known for ellipses has major and minor axes parallel to the $x$ and $y$ axes, and thus no cross term (i.e. $b = 0$). Without rotating the coordinate system there is no hope of writing this as $frac{(x-i)^2}{j} + frac{(y-k)^2}{l} = 1$
$endgroup$
– Arthur
Jan 10 '15 at 1:52
$begingroup$
@Amzoti - Yes, there is the $b^2-4ac$ bit I was talking about. Using that calculator confirmed what I thought, but I still need to prove that $x^2-xy+4y^2-4=0$ represents an ellipse. Also, any non-zero $a$ will yield the same ellipse. This is obvious since you can factor it out from $(1)$.
$endgroup$
– Alec
Jan 10 '15 at 2:14
$begingroup$
@Arthur - You're right. Drawing the equation in Geogebra shows a slightly rotated ellipse. Could $(1)$ after factoring and striking $a neq 0$ be the answer to the problem?
$endgroup$
– Alec
Jan 10 '15 at 2:16
$begingroup$
Why not to use the method described in en.wikipedia.org/wiki/…
$endgroup$
– Claude Leibovici
Jan 10 '15 at 4:23
$begingroup$
For finding the equation in the first place, solving is the system requires a guess about which variable to leave "free"; and, of course, there's all that solving work to do. Using the determinant mentioned in this answer, one generates the equation directly ... albeit with the work of expanding a $6times 6$ determinant.
$endgroup$
– Blue
Aug 18 '18 at 16:04
add a comment |
$begingroup$
The form most known for ellipses has major and minor axes parallel to the $x$ and $y$ axes, and thus no cross term (i.e. $b = 0$). Without rotating the coordinate system there is no hope of writing this as $frac{(x-i)^2}{j} + frac{(y-k)^2}{l} = 1$
$endgroup$
– Arthur
Jan 10 '15 at 1:52
$begingroup$
@Amzoti - Yes, there is the $b^2-4ac$ bit I was talking about. Using that calculator confirmed what I thought, but I still need to prove that $x^2-xy+4y^2-4=0$ represents an ellipse. Also, any non-zero $a$ will yield the same ellipse. This is obvious since you can factor it out from $(1)$.
$endgroup$
– Alec
Jan 10 '15 at 2:14
$begingroup$
@Arthur - You're right. Drawing the equation in Geogebra shows a slightly rotated ellipse. Could $(1)$ after factoring and striking $a neq 0$ be the answer to the problem?
$endgroup$
– Alec
Jan 10 '15 at 2:16
$begingroup$
Why not to use the method described in en.wikipedia.org/wiki/…
$endgroup$
– Claude Leibovici
Jan 10 '15 at 4:23
$begingroup$
For finding the equation in the first place, solving is the system requires a guess about which variable to leave "free"; and, of course, there's all that solving work to do. Using the determinant mentioned in this answer, one generates the equation directly ... albeit with the work of expanding a $6times 6$ determinant.
$endgroup$
– Blue
Aug 18 '18 at 16:04
$begingroup$
The form most known for ellipses has major and minor axes parallel to the $x$ and $y$ axes, and thus no cross term (i.e. $b = 0$). Without rotating the coordinate system there is no hope of writing this as $frac{(x-i)^2}{j} + frac{(y-k)^2}{l} = 1$
$endgroup$
– Arthur
Jan 10 '15 at 1:52
$begingroup$
The form most known for ellipses has major and minor axes parallel to the $x$ and $y$ axes, and thus no cross term (i.e. $b = 0$). Without rotating the coordinate system there is no hope of writing this as $frac{(x-i)^2}{j} + frac{(y-k)^2}{l} = 1$
$endgroup$
– Arthur
Jan 10 '15 at 1:52
$begingroup$
@Amzoti - Yes, there is the $b^2-4ac$ bit I was talking about. Using that calculator confirmed what I thought, but I still need to prove that $x^2-xy+4y^2-4=0$ represents an ellipse. Also, any non-zero $a$ will yield the same ellipse. This is obvious since you can factor it out from $(1)$.
$endgroup$
– Alec
Jan 10 '15 at 2:14
$begingroup$
@Amzoti - Yes, there is the $b^2-4ac$ bit I was talking about. Using that calculator confirmed what I thought, but I still need to prove that $x^2-xy+4y^2-4=0$ represents an ellipse. Also, any non-zero $a$ will yield the same ellipse. This is obvious since you can factor it out from $(1)$.
$endgroup$
– Alec
Jan 10 '15 at 2:14
$begingroup$
@Arthur - You're right. Drawing the equation in Geogebra shows a slightly rotated ellipse. Could $(1)$ after factoring and striking $a neq 0$ be the answer to the problem?
$endgroup$
– Alec
Jan 10 '15 at 2:16
$begingroup$
@Arthur - You're right. Drawing the equation in Geogebra shows a slightly rotated ellipse. Could $(1)$ after factoring and striking $a neq 0$ be the answer to the problem?
$endgroup$
– Alec
Jan 10 '15 at 2:16
$begingroup$
Why not to use the method described in en.wikipedia.org/wiki/…
$endgroup$
– Claude Leibovici
Jan 10 '15 at 4:23
$begingroup$
Why not to use the method described in en.wikipedia.org/wiki/…
$endgroup$
– Claude Leibovici
Jan 10 '15 at 4:23
$begingroup$
For finding the equation in the first place, solving is the system requires a guess about which variable to leave "free"; and, of course, there's all that solving work to do. Using the determinant mentioned in this answer, one generates the equation directly ... albeit with the work of expanding a $6times 6$ determinant.
$endgroup$
– Blue
Aug 18 '18 at 16:04
$begingroup$
For finding the equation in the first place, solving is the system requires a guess about which variable to leave "free"; and, of course, there's all that solving work to do. Using the determinant mentioned in this answer, one generates the equation directly ... albeit with the work of expanding a $6times 6$ determinant.
$endgroup$
– Blue
Aug 18 '18 at 16:04
add a comment |
1 Answer
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$begingroup$
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of b^2+4ac, where I ended up with b^2−4ac=−15a^2<0 which is supposed to be an ellipse.
Yes, it is b^2-4ac actually.
First, solve the determinant
$begin{bmatrix}a & b/2 & d/2\b/2 & c & e/2\d/2 & e/2 & fend{bmatrix}$ = p
Now, if:-
- p=0 and b^2-4ac>0, it is a pair of distinct straight lines.
- p=0 and b^2-4ac=0, it is a either the same line or pair of parrallel lines.
- p=0 and b^2-4ac<0, it is a pair of imaginary lines.
- p≠0 and b^2-4ac=0, it is a parabola.
- p≠0 and b^2-4ac<0, it is an ellipse.
- p≠0 and b^2-4ac>0, it is a hyperbola.
- p≠0, b=0 and a=c, it is a circle.
Sorry, but I'm not that good with MathJax.
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add a comment |
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$begingroup$
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of b^2+4ac, where I ended up with b^2−4ac=−15a^2<0 which is supposed to be an ellipse.
Yes, it is b^2-4ac actually.
First, solve the determinant
$begin{bmatrix}a & b/2 & d/2\b/2 & c & e/2\d/2 & e/2 & fend{bmatrix}$ = p
Now, if:-
- p=0 and b^2-4ac>0, it is a pair of distinct straight lines.
- p=0 and b^2-4ac=0, it is a either the same line or pair of parrallel lines.
- p=0 and b^2-4ac<0, it is a pair of imaginary lines.
- p≠0 and b^2-4ac=0, it is a parabola.
- p≠0 and b^2-4ac<0, it is an ellipse.
- p≠0 and b^2-4ac>0, it is a hyperbola.
- p≠0, b=0 and a=c, it is a circle.
Sorry, but I'm not that good with MathJax.
$endgroup$
add a comment |
$begingroup$
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of b^2+4ac, where I ended up with b^2−4ac=−15a^2<0 which is supposed to be an ellipse.
Yes, it is b^2-4ac actually.
First, solve the determinant
$begin{bmatrix}a & b/2 & d/2\b/2 & c & e/2\d/2 & e/2 & fend{bmatrix}$ = p
Now, if:-
- p=0 and b^2-4ac>0, it is a pair of distinct straight lines.
- p=0 and b^2-4ac=0, it is a either the same line or pair of parrallel lines.
- p=0 and b^2-4ac<0, it is a pair of imaginary lines.
- p≠0 and b^2-4ac=0, it is a parabola.
- p≠0 and b^2-4ac<0, it is an ellipse.
- p≠0 and b^2-4ac>0, it is a hyperbola.
- p≠0, b=0 and a=c, it is a circle.
Sorry, but I'm not that good with MathJax.
$endgroup$
add a comment |
$begingroup$
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of b^2+4ac, where I ended up with b^2−4ac=−15a^2<0 which is supposed to be an ellipse.
Yes, it is b^2-4ac actually.
First, solve the determinant
$begin{bmatrix}a & b/2 & d/2\b/2 & c & e/2\d/2 & e/2 & fend{bmatrix}$ = p
Now, if:-
- p=0 and b^2-4ac>0, it is a pair of distinct straight lines.
- p=0 and b^2-4ac=0, it is a either the same line or pair of parrallel lines.
- p=0 and b^2-4ac<0, it is a pair of imaginary lines.
- p≠0 and b^2-4ac=0, it is a parabola.
- p≠0 and b^2-4ac<0, it is an ellipse.
- p≠0 and b^2-4ac>0, it is a hyperbola.
- p≠0, b=0 and a=c, it is a circle.
Sorry, but I'm not that good with MathJax.
$endgroup$
Can I determine which type of conic section this is? I read somewhere that I can determine it by finding out the sign of b^2+4ac, where I ended up with b^2−4ac=−15a^2<0 which is supposed to be an ellipse.
Yes, it is b^2-4ac actually.
First, solve the determinant
$begin{bmatrix}a & b/2 & d/2\b/2 & c & e/2\d/2 & e/2 & fend{bmatrix}$ = p
Now, if:-
- p=0 and b^2-4ac>0, it is a pair of distinct straight lines.
- p=0 and b^2-4ac=0, it is a either the same line or pair of parrallel lines.
- p=0 and b^2-4ac<0, it is a pair of imaginary lines.
- p≠0 and b^2-4ac=0, it is a parabola.
- p≠0 and b^2-4ac<0, it is an ellipse.
- p≠0 and b^2-4ac>0, it is a hyperbola.
- p≠0, b=0 and a=c, it is a circle.
Sorry, but I'm not that good with MathJax.
answered Aug 18 '18 at 15:54
user79161user79161
191310
191310
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$begingroup$
The form most known for ellipses has major and minor axes parallel to the $x$ and $y$ axes, and thus no cross term (i.e. $b = 0$). Without rotating the coordinate system there is no hope of writing this as $frac{(x-i)^2}{j} + frac{(y-k)^2}{l} = 1$
$endgroup$
– Arthur
Jan 10 '15 at 1:52
$begingroup$
@Amzoti - Yes, there is the $b^2-4ac$ bit I was talking about. Using that calculator confirmed what I thought, but I still need to prove that $x^2-xy+4y^2-4=0$ represents an ellipse. Also, any non-zero $a$ will yield the same ellipse. This is obvious since you can factor it out from $(1)$.
$endgroup$
– Alec
Jan 10 '15 at 2:14
$begingroup$
@Arthur - You're right. Drawing the equation in Geogebra shows a slightly rotated ellipse. Could $(1)$ after factoring and striking $a neq 0$ be the answer to the problem?
$endgroup$
– Alec
Jan 10 '15 at 2:16
$begingroup$
Why not to use the method described in en.wikipedia.org/wiki/…
$endgroup$
– Claude Leibovici
Jan 10 '15 at 4:23
$begingroup$
For finding the equation in the first place, solving is the system requires a guess about which variable to leave "free"; and, of course, there's all that solving work to do. Using the determinant mentioned in this answer, one generates the equation directly ... albeit with the work of expanding a $6times 6$ determinant.
$endgroup$
– Blue
Aug 18 '18 at 16:04