Triangle group is Euclidean if the angle sum is exactly π, spherical if it exceeds π and hyperbolic if it is strictly smaller than π. Moreover, any two triangles with the given angles are congruent. Each triangle group determines a tiling, which is conventionally colored in two colors, so that any two adjacent tiles have opposite colors. In terms of the numbers "l", "m", "n" > 1 there are the following possibilities. formula_4 The triangle group is the infinite symmetry group of a certain tessellation (or tiling) of the Euclidean plane by triangles whose angles add up to π (or 180°). Up
Incircle and excircles of a triangle 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. Thus for example for vertex "B" and adjacent tangencies "T" and "T", The incircle radius is no greater than one-ninth the sum of the altitudes. The squared distance from the incenter "I" to the circumcenter "O" is given by and the distance from the incenter to the center "N" of the nine point circle is The incenter lies in the medial triangle (whose vertices are the midpoints of the sides). The radius of the incircle is