Shape of the universe at least two angles equal 90°, which makes the sum of the 3 angles greater than 180°. An example of a negatively curved surface would be the shape of a saddle or mountain pass. A triangle drawn on a saddle surface will have the sum of the angles adding up to less than 180°. General relativity explains that mass and energy bend the curvature of spacetime and is used to determine what curvature the universe has by using a value called the density parameter, represented with Omega (). The density parameter is the average density of the universe divided by
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