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  2. Internal and external angles - Wikipedia

    en.wikipedia.org/wiki/Internal_and_external_angles

    The formula can be proved by using mathematical induction: starting with a triangle, for which the angle sum is 180°, then replacing one side with two sides connected at another vertex, and so on. The sum of the external angles of any simple polygon, if only one of the two external angles is assumed at each vertex, is 2π radians (360°).

  3. Triangle - Wikipedia

    en.wikipedia.org/wiki/Triangle

    The triangles in both spaces have properties different from the triangles in Euclidean space. For example, as mentioned above, the internal angles of a triangle in Euclidean space always add up to 180°. However, the sum of the internal angles of a hyperbolic triangle is less than 180°, and for any spherical triangle, the sum is more than 180 ...

  4. Incircle and excircles - Wikipedia

    en.wikipedia.org/wiki/Incircle_and_excircles

    Every triangle has three distinct excircles, each tangent to one of the triangle's sides. [3] The center of an excircle is the intersection of the internal bisector of one angle (at vertex , for example) and the external bisectors of the other two.

  5. Thales's theorem - Wikipedia

    en.wikipedia.org/wiki/Thales's_theorem

    Since OA = OB = OC, OBA and OBC are isosceles triangles, and by the equality of the base angles of an isosceles triangle, ∠ OBC = ∠ OCB and ∠ OBA = ∠ OAB. Let α = ∠ BAO and β = ∠ OBC. The three internal angles of the ∆ABC triangle are α, (α + β), and β. Since the sum of the angles of a triangle is equal to 180°, we have

  6. Pentagon - Wikipedia

    en.wikipedia.org/wiki/Pentagon

    An equilateral pentagon is a polygon with five sides of equal length. However, its five internal angles can take a range of sets of values, thus permitting it to form a family of pentagons. In contrast, the regular pentagon is unique up to similarity, because it is equilateral and it is equiangular (its five angles are equal).

  7. Incenter - Wikipedia

    en.wikipedia.org/wiki/Incenter

    The point of intersection of angle bisectors of the 3 angles of triangle ABC is the incenter (denoted by I). The incircle (whose center is I) touches each side of the triangle. In geometry , the incenter of a triangle is a triangle center , a point defined for any triangle in a way that is independent of the triangle's placement or scale.

  8. Decagon - Wikipedia

    en.wikipedia.org/wiki/Decagon

    The triangle has two ... An alternative formula is = where d is the distance ... but vertices alternating two different internal angles.

  9. Angle - Wikipedia

    en.wikipedia.org/wiki/Angle

    In Euclidean geometry, the two acute angles in a right triangle are complementary because the sum of internal angles of a triangle is 180 degrees, and the right angle accounts for 90 degrees. The adjective complementary is from the Latin complementum , associated with the verb complere , "to fill up".