Sum of angles of a triangle The relation between angular defect and the triangle's area was first proven by Johann Heinrich Lambert. One can easily see how hyperbolic geometry breaks Playfair's axiom, Proclus' axiom (the parallelism, defined as non-intersection, is intransitive in an hyperbolic plane), the equidistance postulate (the points on one side of, and equidistant from, a given line do not form a line), and Pythagoras' theorem. A circle cannot have arbitrarily small curvature, so the three points property also fails. The sum of the angles can be arbitrarily small (but positive). For an ideal triangle, a generalization of hyperbolic triangles, this sum is equal
Incircle and excircles of a triangle has area formula_39. Similarly, formula_40 has area formula_41 and formula_42 has area formula_43. Since these three triangles decompose formula_35, we see that the area formula_45 is: where formula_48 is the area of formula_35 and formula_50 is its semiperimeter. For an alternative formula, consider formula_51. This is a right-angled triangle with one side equal to "r" and the other side equal to formula_52. The same is true for formula_53. The large triangle is composed of 6 such triangles and the total area is: The Gergonne triangle (of "ABC") is defined by the 3 touchpoints of the incircle on the 3 sides.