D-Triangle

Let the circles $c_2$ and $c_3'$ used in the construction of the Brocard Points which are tangent to $A_2A_3$ at $A_2$ and $A_3$, respectively, meet again at $D_1$. The points $D_1D_2D_3$ then define the D-triangle. The Vertices of the D-triangle lie on the respective Apollonius Circles.

See also Apollonius Circles, Brocard Points

References

Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, pp. 284-285, 296 and 307, 1929.