# excircle

## excircle

[¦ek′sər·kəl]
(mathematics)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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Since this equicircle lays outside the triangle, it is named an escribed circle, or excircle.
Point D is where the excircle touches the shorter side of the triangle.
"Let the excircle of triangle ABC opposite the vertex A be tangent to the side BC at the point [A.sub.1].
[Table 2-1] Other centers of tetrahedron Position in Position in Centers 2D Geometry 3D Geometry Circumcenter A point where three A point where perpendicular bisectors perpendicular bisecting intersect planes intersect Centroid A point where three A point where median medians intersect planes (Planes with a edge and its opposite edge's middle point) intersect Excenter A point where exterior A point where exterior angle bisectors intersect dihedral-bisecting planes intersect Centers Property Circumcenter Becomes the center of the circumcircle and the circumsphere, respectively Centroid Divides the line which connects a point and the opposite planes' centroid as 2:1, 3:1 respectively Excenter Becomes the center of the excircle and the exosphere, respectively
Where [T.sub.s] denotes the lead of screws, [bar.[phi]] the average helix angle, [R.sub.s] the radius of the excircle, and [R.sub.b] the radius of the radical circle.
Moreover, there are four congruent right triangles in the rectangle ABCD, and each one is matched with a different circle in the dual system, showing it to be an excircle or incircle.
Mack and Czernezkyj carry out a geometric construction of the Pythagorean Tree using each excircle. In three cases they have another PPT, and the sides (a', b', c') of the new triple are simple linear combinations of the sides (a, b, c) of the old.
We wish here to enlarge on the role of the equicircles (incircle and three excircles), and show there is yet another family tree in Pythagoras' garden.
In fact, denoting the diameter of the excircle on the hypotenuse by w, then:
and, since the RHS is always even for PPTs, it follows that the radius of this excircle is also an integer.
The Nagel point is the point of intersection of the line segments from the vertices of the triangle to the points of tangency of the opposite excircles .
The Nagel point N of a triangle ABC is defined as the point of intersection of the cevians AA, BB', and CC', where A', F, and C' are points where the excircles touch the sides (Figure 1).

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