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  2. Special cases of Apollonius' problem - Wikipedia

    en.wikipedia.org/wiki/Special_cases_of_Apollonius...

    In general, the same inversion transforms the given line L and given circle C into two new circles, c 1 and c 2. Thus, the problem becomes that of finding a solution line tangent to the two inverted circles, which was solved above. There are four such lines, and re-inversion transforms them into the four solution circles of the Apollonius problem.

  3. Langley's Adventitious Angles - Wikipedia

    en.wikipedia.org/wiki/Langley's_Adventitious_Angles

    The article contains a history of the problem and a picture featuring the regular triacontagon and its diagonals. In 2015, an anonymous Japanese woman using the pen name "aerile re" published the first known method (the method of 3 circumcenters) to construct a proof in elementary geometry for a special class of adventitious quadrangles problem.

  4. Spherical trigonometry - Wikipedia

    en.wikipedia.org/wiki/Spherical_trigonometry

    Case 3: two sides and an opposite angle given (SSA). The sine rule gives C and then we have Case 7. There are either one or two solutions. Case 4: two angles and an included side given (ASA). The four-part cotangent formulae for sets (cBaC) and (BaCb) give c and b, then A follows from the sine rule. Case 5: two angles and an opposite side given ...

  5. List of incomplete proofs - Wikipedia

    en.wikipedia.org/wiki/List_of_incomplete_proofs

    Yamabe problem. Yamabe claimed a solution in 1960, but Trudinger discovered a gap in 1968, and a complete proof was not given until 1984. Mordell conjecture over function fields. Manin published a proof in 1963, but Coleman (1990) found and corrected a gap in the proof. In 1973 Britton published a 282-page attempted solution of Burnside's problem.

  6. Angle trisection - Wikipedia

    en.wikipedia.org/wiki/Angle_trisection

    The example shows trisection of any angle θ > ⁠ 3π / 4 ⁠ by a ruler with length equal to the radius of the circle, giving trisected angle φ = ⁠ θ / 3 ⁠. Angle trisection is a classical problem of straightedge and compass construction of ancient Greek mathematics.

  7. Ultraparallel theorem - Wikipedia

    en.wikipedia.org/wiki/Ultraparallel_theorem

    Through A' draw a line s' (A'E') on the side closer to E, so that the angle B'A'E' is the same as angle BAE. Then s' meets s in an ordinary point D'. Construct a point D on ray AE so that AD = A'D'. Then D' ≠ D. They are the same distance from r and both lie on s. So the perpendicular bisector of D'D (a segment of s) is also perpendicular to ...

  8. Carnot's theorem (perpendiculars) - Wikipedia

    en.wikipedia.org/wiki/Carnot's_theorem...

    Carnot's theorem: if three perpendiculars on triangle sides intersect in a common point F, then blue area = red area. Carnot's theorem (named after Lazare Carnot) describes a necessary and sufficient condition for three lines that are perpendicular to the (extended) sides of a triangle having a common point of intersection.

  9. Aristotle's axiom - Wikipedia

    en.wikipedia.org/wiki/Aristotle's_axiom

    If ^ is an acute angle and AB is any segment, then there exists a point P on the ray and a point Q on the ray , such that PQ is perpendicular to OX and PQ > AB. Aristotle's axiom is a consequence of the Archimedean property , [ 1 ] and the conjunction of Aristotle's axiom and the Lotschnittaxiom , which states that "Perpendiculars raised on ...