In Euclidean geometry, the extouch triangle of a

triangle A triangle is a polygon with three Edge (geometry), edges and three Vertex (geometry), vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC. In Euclidean geometry, an ...

is formed by joining the points at which the three excircles touch the triangle.

Coordinates

The vertices of the extouch triangle are given in

trilinear coordinates In geometry, the trilinear coordinates of a point relative to a given triangle describe the relative directed distances from the three sidelines of the triangle. Trilinear coordinates are an example of homogeneous coordinates. The ratio is t ...

by: :

\begin
T_A &= 0 : \csc^2 : \csc^2\\
T_B &= \csc^2 : 0 : \csc^2\\
T_C &= \csc^2 : \csc^2 : 0
\end

or equivalently, where are the lengths of the sides opposite angles respectively, :

\begin
T_A &= 0 : \frac : \frac \\
T_B &= \frac : 0 : \frac \\
T_C &= \frac :  \frac : 0.
\end

Related figures

The triangle's splitters are lines connecting the vertices of the original triangle to the corresponding vertices of the extouch triangle; they bisect the triangle's perimeter and meet at the Nagel point. This is shown in blue and labelled "N" in the diagram. The

Mandart inellipse In geometry, the Mandart inellipse of a triangle is an ellipse inscribed within the triangle, tangent to its sides at the contact points of its excircles (which are also the vertices of the extouch triangle and the endpoints of the splitters). ...

is tangent to the sides of the reference triangle at the three vertices of the extouch triangle.

Area

The area of the extouch triangle, , is given by: :

K_T= K\frac

where and are the area and radius of the incircle, is the

semiperimeter In geometry, the semiperimeter of a polygon is half its perimeter. Although it has such a simple derivation from the perimeter, the semiperimeter appears frequently enough in formulas for triangles and other figures that it is given a separate name ...

of the original triangle, and are the side lengths of the original triangle. This is the same area as that of the

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

.Weisstein, Eric W. "Extouch Triangle." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/ExtouchTriangle.html

References

{{reflist Circles Objects defined for a triangle