In elementary plane geometry, the **power of a point** is a real number that reflects the relative distance of a given point from a given circle. It was introduced by Jakob Steiner 1826.^{[1]}

Specifically, the power of a point with respect to a circle with center and radius is defined by

If is *outside* the circle, then ,

if is *on* the circle, then and

if is *inside* the circle, then .

Due to the theorem of Pythagoras the number has the simple geometric meanings shown in the diagram: For a point outside the circle is the squared tangential distance of point to the circle .

Points with equal power, isolines of , are circles concentric to circle .

Steiner used the power of a point for proofs of several statements on circles, for example:

- Determination of a circle, that intersects four circles by the same angle.
^{[2]} - Solving the Problem of Apollonius
- Construction of the Malfatti circles:
^{[3]}For a given triangle determine three circles, which touch each other and two sides of the triangle each. - Spherical version of Malfatti's problem:
^{[4]}The triangle is a spherical one.

Essential tools for invstigations on circles are the radical axis of two circles and the radical center of three circles.

The power diagram of a set of circles divides the plane into regions within which the circle minimizing the power is constant.

More generally, French mathematician Edmond Laguerre defined the power of a point with respect to any algebraic curve in a similar way.

## Geometric properties

Besides the properties mentioned in the lead there are further properties:

### Orthogonal circle

For any point *outside* of the circle there are two tangent points on circle , which have equal distance to . Hence the circle with center through passes , too, and intersects orthogonal:

- The circle with center and radius intersects circle
*orthogonal*.

If the radius of the circle centered at is different from one gets the angle of intersection between the two circles applying the Law of cosines (see the diagram):

( and are normals to the circle tangents.)

If lies inside the blue circle, then and is always different from .

If the angle is given, then one gets the radius by solving the quadratic equation

- .

### Intersecting secants theorem, intersecting chords theorem

For the *intersecting secants theorem* and *chord theorem* the power of a point plays the role of an invariant:

*Intersecting secants theorem*: For a point*outside*a circle and the intersection points of a secant line with the following statement is true: , hence the product is independent of line . If is tangent then and the statement is the*tangent-secant theorem*.*Intersecting chords theorem*: For a point*inside*a circle and the intersection points of a secant line with the following statement is true: , hence the product is independent of line .

### Radical axis

Let be a point and two non concentric circles with
centers and radii . Point has the power with respect to circle . The set of all points with is a line called *radical axis*. It contains possible common points of the circles and is perpendicular to line .

## Secants theorem, chords theorem: common proof

Both theorems, including the *tangent-secant theorem*, can be proven uniformly:

Let be a point, a circle with the origin as its center and an arbitrary unit vector. The parameters of possible common points of line (through ) and circle can be determined by inserting the parmetric equation into the circle's equation:

From Vieta's theorem one finds:

- . (independent of !)

is the power of with respect for circle .

Because of one gets the following statement for the points :

- , if is outside the circle,
- , if is inside the circle ( have different signs !).

In case of line is a tangent and the square of the tangential distance of point to circle .

## Similarity points, common power of two circles

### Similarity points

Similarity points are an essential tool for Steiner's investigations on circles.^{[5]}

Given two circles

- .

A homothety (similarity) , that maps onto streches (jolts) radius to and has its center on the line , because . If center is between the scale factor is . In the other case . In any case:

- .

Inserting and solving for yields:

- .

Point

is called *exterior similarity point* and

*inner similarity point*.

In case of one gets .

In case of : is the point at infinity of line and is the center of .

In case of the circles touch each other at point *inside* (both circles on the same side of the common tangent line).

In case of the circles touch each other at point *outside* (both circles on different sides of the common tangent line).

Further more:

- If the circles lie
*disjoint*(the discs have no points in common), the outside common tangents meet at and the inner ones at . - If one circle is contained
*within the other*, the points lie*within*both circles. - The pairs are projective harmonic conjugate: Their cross ratio is .

Monge's theorem states: The *outer* similarity points of three disjoint circles lie on a line.

### Common power of two circles

Let be two circles, their outer similarity point and a line through , which meets the two circles at four points . From the defining property of point one gets

and from the secant theorem (see above) the two equations

Combining these three equations yields:

Hence:

- (independent of line !).

The analog statement for the inner similarity point is true, too.

The invariants are called by Steiner *common power of the two circles* (*gemeinschaftliche Potenz der beiden Kreise bezüglich ihrer Ähnlichkeitspunkte*).^{[6]}

The pairs and of points are *antihomologous* points. The pairs and are *homologous*.^{[7]}^{[8]}

### Determination of a circle, that is tangent to two circles

For a second secant through :

From the secant theorem one gets:

- The four points lie on a circle.

And analogously:

- The four points lie on a circle, too.

Because the radical lines of three circles meet at the radical (see: article radical line), one gets:

- The secants meet on the radical axis of the given two circles.

Moving the lower secant (see diagram) towards the upper one, the red circle becomes a circle, that is tangent to both given circles. The center of the tangent circle is the intercept of the lines . The secants become tangents at the points . The tangents intercept at the radical line (in the diagram yellow).

Similar considerations generate the second tangent circle, that meets the given circles at the points (see diagram).

All tangent circles to the given circles can be found by varying line .

- Positions of the centers

If is the center and the radius of the circle, that is tangent to the given circles at the points , then:

Hence: the centers lie on a hyperbola with

- foci ,
- distance of the vertices ,
- center is the center of ,
- linear eccentricity und
- .

Considerations on the outside tangent circles lead to the analog result:

If is the center and the radius of the circle, that is tangent to the given circles at the points , then:

The centers lie on the same hyperbola, but on the right branch.

See also Problem of Apollonius.

## Power with respect to a sphere

The idea of the power of a point with repect to a circle can be extended to a sphere
.^{[9]} The secants and chords theorems are true for a sphere, too, and can be proven literally as in the circle case.

## Darboux product

The power of a point is a special case of the Darboux product between two circles, which is given by^{[10]}

where *A*_{1} and *A*_{2} are the centers of the two circles and *r*_{1} and *r*_{2} are their radii. The power of a point arises in the special case that one of the radii is zero.

If the two circles are orthogonal, the Darboux product vanishes.

If the two circles intersect, then their Darboux product is

where *φ* is the angle of intersection (see section *orthogonal circle*).

## Laguerre's theorem

Laguerre defined the power of a point *P* with respect to an algebraic curve of degree *n* to be the product of the distances from the point to the intersections of a circle through the point with the curve, divided by the *n*th power of the diameter *d*. Laguerre showed that this number is independent of the diameter (Laguerre 1905). In the case when the algebraic curve is a circle this is not quite the same as the power of a point with respect to a circle defined in the rest of this article, but differs from it by a factor of *d*^{2}.

## References

**^**Jakob Steiner:*Einige geometrische Betrachtungen*, 1826, S. 164**^**Steiner, p. 163**^**Steiner, p. 178**^**Steiner, p. 182**^**Steiner: p. 170,171**^**Steiner: p. 175**^**Michel Chasles, C. H. Schnuse:*Die Grundlehren der neuern Geometrie, erster Theil*, Verlag Leibrock, Braunschweig, 1856, p. 312**^**William J. M'Clelland:*A Treatise on the Geometry of the Circle and Some Extensions to Conic Sections by the Method of Reciprocation*,1891, Verlag: Creative Media Partners, LLC,ISBN: 9780344903748, p. 121,220**^**K.P. Grothemeyer:*Analytische Geometrie*, Sammlung Göschen 65/65A, Berlin 1962, S. 54**^**Pierre Larochelle, J. Michael McCarthy:*Proceedings of the 2020 USCToMM Symposium on Mechanical Systems and Robotics*, 2020, Springer-Verlag, ISBN 978-3-030-43929-3, p. 97

- Coxeter, H. S. M. (1969),
*Introduction to Geometry*(2nd ed.), New York: Wiley. - Darboux, Gaston (1872), "Sur les relations entre les groupes de points, de cercles et de sphéres dans le plan et dans l'espace",
*Annales Scientifiques de l'École Normale Supérieure*,**1**: 323–392. - Laguerre, Edmond (1905),
*Oeuvres de Laguerre: Géométrie*(in French), Gauthier-Villars et fils, p. 20 - Steiner, Jakob (1826), "Einige geometrische Betrachtungen",
*Journal für die reine und angewandte Mathematik*,**1**: 161–184. - Berger, Marcel (1987),
*Geometry I*, Springer, ISBN 978-3-540-11658-5

## Further reading

- Ogilvy C. S. (1990),
*Excursions in Geometry*, Dover Publications, pp. 6–23, ISBN 0-486-26530-7 - Coxeter H. S. M., Greitzer S. L. (1967),
*Geometry Revisited*, Washington: MAA, pp. 27–31, 159–160, ISBN 978-0-88385-619-2 - Johnson RA (1960),
*Advanced Euclidean Geometry: An elementary treatise on the geometry of the triangle and the circle*(reprint of 1929 edition by Houghton Miflin ed.), New York: Dover Publications, pp. 28–34, ISBN 978-0-486-46237-0

## External links

Wikimedia Commons has media related to Power of a point. |

- Jacob Steiner and the Power of a Point at Convergence
- Weisstein, Eric W. "Circle Power".
*MathWorld*. - Intersecting Chords Theorem at cut-the-knot
- Intersecting Chords Theorem With interactive animation
- Intersecting Secants Theorem With interactive animation