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  2. Generalized trigonometry - Wikipedia

    en.wikipedia.org/wiki/Generalized_trigonometry

    Ordinary trigonometry studies triangles in the Euclidean plane ⁠ ⁠.There are a number of ways of defining the ordinary Euclidean geometric trigonometric functions on real numbers, for example right-angled triangle definitions, unit circle definitions, series definitions [broken anchor], definitions via differential equations [broken anchor], and definitions using functional equations.

  3. No-three-in-line problem - Wikipedia

    en.wikipedia.org/wiki/No-three-in-line_problem

    This application was the motivation for Paul Erdős to find his solution for the no-three-in-line problem. [13] It remained the best area lower bound known for the Heilbronn triangle problem from 1951 until 1982, when it was improved by a logarithmic factor using a construction that was not based on the no-three-in-line problem. [14]

  4. Law of cosines - Wikipedia

    en.wikipedia.org/wiki/Law_of_cosines

    Given triangle sides b and c and angle γ there are sometimes two solutions for a. The theorem is used in solution of triangles , i.e., to find (see Figure 3): the third side of a triangle if two sides and the angle between them is known: c = a 2 + b 2 − 2 a b cos ⁡ γ ; {\displaystyle c={\sqrt {a^{2}+b^{2}-2ab\cos \gamma }}\,;}

  5. Simplex - Wikipedia

    en.wikipedia.org/wiki/Simplex

    In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension.

  6. Langley's Adventitious Angles - Wikipedia

    en.wikipedia.org/wiki/Langley's_Adventitious_Angles

    adventitious quadrangles problem. A quadrilateral such as BCEF is called an adventitious quadrangle when the angles between its diagonals and sides are all rational angles, angles that give rational numbers when measured in degrees or other units for which the whole circle is a rational number.

  7. 5-simplex - Wikipedia

    en.wikipedia.org/wiki/5-simplex

    It has six vertices, 15 edges, 20 triangle faces, 15 tetrahedral cells, and 6 5-cell facets. It has a dihedral angle of cos −1 ( ⁠ 1 / 5 ⁠ ), or approximately 78.46°. The 5-simplex is a solution to the problem: Make 20 equilateral triangles using 15 matchsticks, where each side of every triangle is exactly one matchstick.

  8. Pythagorean theorem - Wikipedia

    en.wikipedia.org/wiki/Pythagorean_theorem

    Generalization of Pythagoras' theorem by Tâbit ibn Qorra. [46] Lower panel: reflection of triangle CAD (top) to form triangle DAC, similar to triangle ABC (top). At any selected angle of a general triangle of sides a, b, c, inscribe an isosceles triangle such that the equal angles at its base θ are the same as the selected angle.

  9. Simson line - Wikipedia

    en.wikipedia.org/wiki/Simson_line

    The Simson line of a vertex of the triangle is the altitude of the triangle dropped from that vertex, and the Simson line of the point diametrically opposite to the vertex is the side of the triangle opposite to that vertex. If P and Q are points on the circumcircle, then the angle between the Simson lines of P and Q is half the angle of the ...