# Incircle and excircles of a triangle

**Incircle**redirects here. For incircles of non-triangle polygons, see Tangential quadrilateral or Tangential polygon.

In geometry, the **incircle** or **inscribed circle** of a triangle is the largest circle contained in the triangle; it touches (is tangent to) the three sides. The center of the incircle is called the triangle's **incenter**.

An **excircle** or **escribed circle** of the triangle is a circle lying outside the triangle, tangent to one of its sides and tangent to the extensions of the other two.
Every triangle has three distinct excircles, each tangent to one of the triangle's sides.

The center of the incircle, called the incenter, can be found as the intersection of the three internal angle bisectors. The center of an excircle is the intersection of the internal bisector of one angle and the external bisectors of the other two. Because the internal bisector of an angle is perpendicular to its external bisector, it follows that the center of the incircle together with the three excircle centers form an orthocentric system.^{[1]}^{:p. 182}

Polygons with more than three sides do not all have an incircle tangent to all sides; those that do are called tangential polygons. See also Tangent lines to circles.

## Contents

## Relation to area of the triangle

The radii of the incircles and excircles are closely related to the area of the triangle.^{[2]}

### Incircle

Suppose has an incircle with radius *r* and center *I*.
Let *a* be the length of *BC*, *b* the length of *AC*, and *c* the length of *AB*.
Now, the incircle is tangent to *AB* at some point *C′*, and so
is right.
Thus the radius *C'I* is an altitude of
.
Therefore
has base length *c* and height *r*, and so has area
.
Similarly,
has area
and
has area .
Since these three triangles decompose , we see that

where is the area of and is its semiperimeter.

For an alternative formula, consider . This is a right-angled triangle with one side equal to *r* and the other side equal to . The same is true for . The large triangle is composed of 6 such triangles and the total area is:

### Excircles

The radii in the excircles are called the **exradii**. Let the excircle at side *AB* touch at side *AC* extended at *G*, and let this excircle's
radius be and its center be . Then is an altitude of ,
so has area . By a similar argument,
has area
and
has area
.
Thus

So, by symmetry,

By the law of cosines, we have

Combining this with the identity , we have

which is Heron's formula.

From these formulas one can see that the excircles are always larger than the incircle and that the largest excircle is the one tangent to the longest side and the smallest excircle is tangent to the shortest side. Further, combining these formulas yields:^{[3]}

The ratio of the area of the incircle to the area of the triangle is less than or equal to , with equality holding only for equilateral triangles.^{[4]}

## Related constructions

### Nine-point circle and Feuerbach point

The circle tangent to all three of the excircles as well as the incircle is known as the nine-point circle. The point where the nine-point circle touches the incircle is known as the Feuerbach point.

### Gergonne triangle and point

The **Gergonne triangle** (of *ABC*) is defined by the 3 touchpoints of the incircle on the 3 sides.
Those vertices are denoted as *T _{A}*, etc.
The point that

*T*denotes, lies opposite to

_{A}*A*.

This **Gergonne triangle** *T _{A}T_{B}T_{C}* is also known as the

**contact triangle**or

**intouch triangle**of

*ABC*.

The three lines *AT _{A}*,

*BT*and

_{B}*CT*intersect in a single point called

_{C}**Gergonne point**, denoted as

*Ge*-

*X(7)*. The Gergonne point lies in the open orthocentroidal disk punctured at its own center, and could be any point therein.

^{[5]}

Interestingly, the Gergonne point of a triangle is the symmedian point of the Gergonne triangle. For a full set of properties of the Gergonne point see.^{[6]}

Trilinear coordinates for the vertices of the intouch triangle are given by

Trilinear coordinates for the Gergonne point are given by

or, equivalently, by the Law of Sines,

### Nagel triangle and point

The **Nagel triangle** of *ABC* is denoted by the vertices *X _{A}*,

*X*and

_{B}*X*that are the three points where the excircles touch the reference triangle

_{C}*ABC*and where

*X*is opposite of

_{A}*A*, etc. This triangle

*X*is also known as the

_{A}X_{B}X_{C}**extouch triangle**of

*ABC*. The circumcircle of the extouch triangle

*X*is called the

_{A}X_{B}X_{C}**Mandart circle**. The three lines

*AX*,

_{A}*BX*and

_{B}*CX*are called the splitters of the triangle; they each bisect the perimeter of the triangle, and they intersect in a single point, the triangle's Nagel point

_{C}*Na*-

*X(8)*.

Trilinear coordinates for the vertices of the extouch triangle are given by

Trilinear coordinates for the Nagel point are given by

or, equivalently, by the Law of Sines,

It is the isotomic conjugate of the Gergonne point.

### Incentral and excentral triangles

The points of intersection of the interior angle bisectors of *ABC* with the segments *BC,CA,AB* are the vertices of the **incentral triangle**.

Trilinear coordinates for the vertices of the incentral triangle are given by

Trilinear coordinates for the vertices of the excentral triangle are given by

## Equations for four circles

Let x : y : z be a variable point in trilinear coordinates, and let u = cos^{2}*(A/2)*, v = cos^{2}*(B/2)*, w = cos^{2}*(C/2)*. The four circles described above are given equivalently by either of the two given equations:^{[7]}^{:p. 210-215}

## Euler's theorem

Euler's theorem states that in a triangle:

where *R* and *r*_{in} are the circumradius and inradius respectively, and *d* is the distance between the circumcenter and the incenter.

For excircles the equation is similar:

where *r*_{ex} is the radius of one of the excircles, and *d* is the distance between the circumcenter and this excircle's center.
^{[8]}
^{[9]}
^{[10]}

## Other incircle properties

Suppose the tangency points of the incircle divide the sides into lengths of *x* and *y*, *y* and *z*, and *z* and *x*. Then the incircle has the radius^{[11]}

and the area of the triangle is

If the altitudes from sides of lengths *a*, *b*, and *c* are *h _{a}*,

*h*, and

_{b}*h*then the inradius

_{c}*r*is one-third of the harmonic mean of these altitudes, i.e.

The product of the incircle radius *r* and the circumcircle radius *R* of a triangle with sides *a*, *b*, and *c* is^{[1]}^{:p. 189, #298(d)}

Some relations among the sides, incircle radius, and circumcircle radius are:^{[12]}

Any line through a triangle that splits both the triangle's area and its perimeter in half goes through the triangle's incenter (the center of its incircle). There are either one, two, or three of these for any given triangle.^{[13]}

Denoting the center of the incircle of triangle *ABC* as *I*, we have^{[14]}

and^{[15]}^{:p.121,#84}

The distance from any vertex to the incircle tangency on either adjacent side is half the sum of the vertex's adjacent sides minus half the opposite side.^{[16]} Thus for example for vertex *B* and adjacent tangencies *T*_{A}* and *T* _{}*C,

The incircle radius is no greater than one-ninth the sum of the altitudes.^{[17]}^{:p. 289}

The squared distance from the incenter *I* to the circumcenter *O* is given by^{[18]}^{:p.232}

and the distance from the incenter to the center *N* of the nine point circle is^{[18]}^{:p.232}

The incenter lies in the medial triangle (whose vertices are the midpoints of the sides).^{[18]}^{:p.233, Lemma 1}

## Other excircle properties

The circular hull of the excircles is internally tangent to each of the excircles, and thus is an Apollonius circle.^{[19]} The radius of this Apollonius circle is where *r* is the incircle radius and *s* is the semiperimeter of the triangle.^{[20]}

The following relations hold among the inradius *r*, the circumradius *R*, the semiperimeter *s*, and the excircle radii *r*_{'a}*, *r* _{}*b

*,*r

*c*

_{}*:*

^{[12]}The circle through the centers of the three excircles has radius 2*R*.^{[12]}

If *H* is the orthocenter of triangle *ABC*, then^{[12]}

## Generalization to other polygons

Some (but not all) quadrilaterals have an incircle. These are called tangential quadrilaterals. Among their many properties perhaps the most important is that their opposite sides have equal sums. This is called the Pitot theorem.

More generally, a polygon with any number of sides that has an inscribed circle—one that is tangent to each side—is called a tangential polygon.

## See also

- Altitude (triangle)
- Circumscribed circle
- Ex-tangential quadrilateral
- Harcourt's theorem
- Inconic
- Inscribed sphere
- Power of a point
- Steiner inellipse
- Tangential quadrilateral
- Triangle center

## References

- ↑
^{1.0}^{1.1}Johnson, Roger A.,*Advanced Euclidean Geometry*, Dover, 2007 (orig. 1929). - ↑ Coxeter, H.S.M. "Introduction to Geometry
*2nd ed. Wiley, 1961.* - ↑ Baker, Marcus, "A collection of formulae for the area of a plane triangle,"
*Annals of Mathematics*, part 1 in vol. 1(6), January 1885, 134-138. (See also part 2 in vol. 2(1), September 1885, 11-18.) - ↑ Minda, D., and Phelps, S., "Triangles, ellipses, and cubic polynomials",
*American Mathematical Monthly*115, October 2008, 679-689: Theorem 4.1. - ↑ Christopher J. Bradley and Geoff C. Smith, "The locations of triangle centers",
*Forum Geometricorum*6 (2006), 57--70. http://forumgeom.fau.edu/FG2006volume6/FG200607index.html - ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}
- ↑ Whitworth, William Allen.
*Trilinear Coordinates and Other Methods of Modern Analytical Geometry of Two Dimensions*, Forgotten Books, 2012 (orig. Deighton, Bell, and Co., 1866). http://www.forgottenbooks.com/search?q=Trilinear+coordinates&t=books - ↑ Nelson, Roger, "Euler's triangle inequality via proof without words,"
*Mathematics Magazine*81(1), February 2008, 58-61. - ↑ Johnson, R. A.
*Modern Geometry*, Houghton Mifflin, Boston, 1929: p. 187. - ↑ Emelyanov, Lev, and Emelyanova, Tatiana. "Euler’s formula and Poncelet’s porism",
*Forum Geometricorum*1, 2001: pp. 137–140. - ↑ Chu, Thomas,
*The Pentagon*, Spring 2005, p. 45, problem 584. - ↑
^{12.0}^{12.1}^{12.2}^{12.3}Bell, Amy, "Hansen’s right triangle theorem, its converse and a generalization",*Forum Geometricorum*6, 2006, 335–342. - ↑ Kodokostas, Dimitrios, "Triangle Equalizers,"
*Mathematics Magazine*83, April 2010, pp. 141-146. - ↑ Allaire, Patricia R.; Zhou, Junmin; and Yao, Haishen, "Proving a nineteenth century ellipse identity",
*Mathematical Gazette*96, March 2012, 161-165. - ↑ Altshiller-Court, Nathan.
*College Geometry*, Dover Publications, 1980. - ↑
*Mathematical Gazette*, July 2003, 323-324. - ↑ Posamentier, Alfred S., and Lehmann, Ingmar.
*The Secrets of Triangles*, Prometheus Books, 2012. - ↑
^{18.0}^{18.1}^{18.2}{{#invoke:Citation/CS1|citation |CitationClass=journal }}. - ↑ Grinberg, Darij, and Yiu, Paul, "The Apollonius Circle as a Tucker Circle",
*Forum Geometricorum*2, 2002: pp. 175-182. - ↑ Stevanovi´c, Milorad R., "The Apollonius circle and related triangle centers",
*Forum Geometricorum*3, 2003, 187-195.

- Clark Kimberling, "Triangle Centers and Central Triangles,"
*Congressus Numerantium*129 (1998) i-xxv and 1-295. - Sándor Kiss, "The Orthic-of-Intouch and Intouch-of-Orthic Triangles,"
*Forum Geometricorum*6 (2006) 171-177.

## External links

- Derivation of formula for radius of incircle of a triangle
- Weisstein, Eric W., "Incircle",
*MathWorld*.

### Interactive

- Triangle incenter Triangle incircle Incircle of a regular polygon With interactive animations
- Constructing a triangle's incenter / incircle with compass and straightedge An interactive animated demonstration
- Equal Incircles Theorem at cut-the-knot
- Five Incircles Theorem at cut-the-knot
- Pairs of Incircles in a Quadrilateral at cut-the-knot
- An interactive Java applet for the incenter