In a Euclidean space, the sum of angles of a triangle equals a straight angle (180 degrees, π radians, two right angles, or a half-turn). A triangle has three angles, and has one at each vertex, bounded by a pair of adjacent sides. The sum can be computed directly using the definition of angle based on the dot product and trigonometric identities, or more quickly by reducing to the two-dimensional case and using Euler's identity. It was unknown for a long time whether other geometries exist, for which this sum is different. The influence of this problem on mathematics was particularly strong during the 19th century. Ultimately, the answer was proven to be positive: in other spaces (geometries) this sum can be greater or lesser, but it then must depend on the triangle. Its difference from 180° is a case of angular defect and serves as an important distinction for geometric systems.
Cases
Euclidean geometry In Euclidean geometry, the triangle postulate states that the sum of the angles of a triangle is two right angles. This postulate is equivalent to the parallel postulate. In the presence of the other axioms of Euclidean geometry, the following statements are equivalent:
Triangle postulate: The sum of the angles of a triangle is two right angles. Playfair's axiom: Given a straight line and a point not on the line, exactly one straight line may be drawn through the point parallel to the given line. Proclus' axiom: If a line intersects one of two parallel lines, it must intersect the other also. Equidistance postulate: Parallel lines are everywhere equidistant (i.e. the distance from each point on one line to the other line is always the same.) Triangle area property: The area of a triangle can be as large as we please. Three points property: Three points either lie on a line or lie on a circle. Pythagoras' theorem: In a right-angled triangle, the square of the hypotenuse equals the sum of the squares of the other two sides.
Spherical geometry
Spherical geometry does not satisfy several of Euclid's axioms, including the parallel postulate. In addition, the sum of angles is not 180°. For a spherical triangle, the sum of the angles is greater than 180° and can be up to 540°. The amount by which the sum of the angles exceeds 180° is called the spherical excess, denoted as E {\textstyle E} or Δ {\textstyle \Delta } . The spherical excess and the area A {\textstyle A} of the triangle determine each other via the relation (called Girard's theorem): E = A r 2 {\displaystyle E={\frac {A}{r^{2}}}} where r {\displaystyle r} is the radius of the sphere, equal to r = 1 κ {\textstyle r={\frac {1}{\sqrt {\kappa }}}} where κ > 0 {\textstyle \kappa >0} is the constant curvature. The spherical excess can also be calculated from the three side lengths, the lengths of two sides and their angle, or the length of one side and the two adjacent angles (see spherical trigonometry). In the limit where the three side lengths tend to 0 {\displaystyle 0} , the spherical excess also tends to 0 {\displaystyle 0} : the spherical geometry locally resembles the Euclidean one. More generally, the Euclidean law is recovered as a limit when the area tends to 0 {\displaystyle 0} (which does not imply that the side lengths do so).
A spherical triangle is determined up to isometry by E {\textstyle E} , one side length and one adjacent angle. More precisely, according to Lexell's theorem, given a spherical segment [ A , B ] {\textstyle [A,B]} as a fixed side and a number 0 ∘ < E < 360 ∘ {\textstyle 0^{\circ }<E<360^{\circ }} , the set of points C {\textstyle C} such that the triangle A B C {\textstyle ABC} has spherical excess E {\displaystyle E} is a circle through the antipodes A ′ , B ′ {\textstyle A',B'} of A {\textstyle A} and B {\textstyle B} . Hence, the level sets of E {\textstyle E} form a foliation of the sphere with two singularities A ′ , B ′ {\displaystyle A',B'} , and the gradient vector of E {\textstyle E} is orthogonal to this foliation.
Hyperbolic geometry
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