Difference between revisions of "Degree of a point"
From Encyclopedia of Mathematics
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| − | '' | + | {{TEX|done}} |
| + | ''$M_0=(x_0,y_0)$ relative to a circle'' | ||
| − | + | $$(x-a)^2+(y-b)^2=R^2$$ | |
| − | with centre at a point | + | ''with centre at a point $(a,b)$'' |
The number | The number | ||
| − | + | $$p=(x_0-a)^2+(y_0-b)^2-R^2.$$ | |
| − | One has < | + | One has $p<0$ if $M_0$ lies within the circle; $p=0$ if $M_0$ lies on the circle; $p>0$ if $M_0$ lies outside the circle. The degree of $M_0$ relative to a circle can be represented as the product of the vectors $\overrightarrow{M_0M_1}$ and $\overrightarrow{M_0M_2}$, where $M_1$ and $M_2$ are the points of intersection of the circle and an arbitrary straight line passing through $M_0$. In particular, the degree of a point $M_0$ relative to a circle is equal to the square of the length of the tangent drawn from $M_0$ to the circle. |
The set of all circles in the plane relative to which a given point has an identical degree forms a [[Net|net]] of circles. The set of points of identical degree relative to two non-concentric circles forms a radical axis. | The set of all circles in the plane relative to which a given point has an identical degree forms a [[Net|net]] of circles. The set of points of identical degree relative to two non-concentric circles forms a radical axis. | ||
The degree of a point relative to a sphere is defined in the same way. The set of all spheres relative to which a given point has identical degree is called a [[Web of spheres|web of spheres]]. The set of all spheres relative to which the points of a straight line (the radical axis) have identical degree (different for different points) forms a [[Net|net]] of spheres. The set of all spheres relative to which the points of a plane (the radical plane) have identical degree (different for different points) forms a [[Bundle|bundle]] of spheres. | The degree of a point relative to a sphere is defined in the same way. The set of all spheres relative to which a given point has identical degree is called a [[Web of spheres|web of spheres]]. The set of all spheres relative to which the points of a straight line (the radical axis) have identical degree (different for different points) forms a [[Net|net]] of spheres. The set of all spheres relative to which the points of a plane (the radical plane) have identical degree (different for different points) forms a [[Bundle|bundle]] of spheres. | ||
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====Comments==== | ====Comments==== | ||
| − | Customarily this notion is called the power of the point | + | Customarily this notion is called the power of the point $M_0$ relative to the circle $(x-a)^2+(y-b)^2=R^2$. |
====References==== | ====References==== | ||
| − | + | * {{Ref|a1}} J.L. Coolidge, "A treatise on the circle and the sphere" , Clarendon Press (1916) {{ZBL|46.0921.02}} | |
Latest revision as of 06:25, 12 October 2023
$M_0=(x_0,y_0)$ relative to a circle
$$(x-a)^2+(y-b)^2=R^2$$
with centre at a point $(a,b)$
The number
$$p=(x_0-a)^2+(y_0-b)^2-R^2.$$
One has $p<0$ if $M_0$ lies within the circle; $p=0$ if $M_0$ lies on the circle; $p>0$ if $M_0$ lies outside the circle. The degree of $M_0$ relative to a circle can be represented as the product of the vectors $\overrightarrow{M_0M_1}$ and $\overrightarrow{M_0M_2}$, where $M_1$ and $M_2$ are the points of intersection of the circle and an arbitrary straight line passing through $M_0$. In particular, the degree of a point $M_0$ relative to a circle is equal to the square of the length of the tangent drawn from $M_0$ to the circle.
The set of all circles in the plane relative to which a given point has an identical degree forms a net of circles. The set of points of identical degree relative to two non-concentric circles forms a radical axis.
The degree of a point relative to a sphere is defined in the same way. The set of all spheres relative to which a given point has identical degree is called a web of spheres. The set of all spheres relative to which the points of a straight line (the radical axis) have identical degree (different for different points) forms a net of spheres. The set of all spheres relative to which the points of a plane (the radical plane) have identical degree (different for different points) forms a bundle of spheres.
Comments
Customarily this notion is called the power of the point $M_0$ relative to the circle $(x-a)^2+(y-b)^2=R^2$.
References
- [a1] J.L. Coolidge, "A treatise on the circle and the sphere" , Clarendon Press (1916) Zbl 46.0921.02
How to Cite This Entry:
Degree of a point. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Degree_of_a_point&oldid=14658
Degree of a point. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Degree_of_a_point&oldid=14658
This article was adapted from an original article by A.B. Ivanov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article