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  2. Nine-point circle - Wikipedia

    en.wikipedia.org/wiki/Nine-point_circle

    The nine-point circles are all congruent with a radius of half that of the cyclic quadrilateral's circumcircle. The nine-point circles form a set of four Johnson circles. Consequently, the four nine-point centers are cyclic and lie on a circle congruent to the four nine-point circles that is centered at the anticenter of the cyclic quadrilateral.

  3. Category:Theorems about circles - Wikipedia

    en.wikipedia.org/.../Category:Theorems_about_circles

    Download as PDF; Printable version; In other projects Wikidata item; ... Pages in category "Theorems about circles" The following 21 pages are in this category, out ...

  4. Clifford's circle theorems - Wikipedia

    en.wikipedia.org/wiki/Clifford's_circle_theorems

    The second theorem considers five circles in general position passing through a single point M. Each subset of four circles defines a new point P according to the first theorem. Then these five points all lie on a single circle C. The third theorem considers six circles in general position that pass through a single point M. Each subset of five ...

  5. Feuerbach point - Wikipedia

    en.wikipedia.org/wiki/Feuerbach_point

    The nine-point circle passes through these three midpoints; thus, it is the circumcircle of the medial triangle. These two circles meet in a single point, where they are tangent to each other. That point of tangency is the Feuerbach point of the triangle. Associated with the incircle of a triangle are three more circles, the excircles. These ...

  6. Thales's theorem - Wikipedia

    en.wikipedia.org/wiki/Thales's_theorem

    In geometry, Thales's theorem states that if A, B, and C are distinct points on a circle where the line AC is a diameter, the angle ∠ ABC is a right angle. Thales's theorem is a special case of the inscribed angle theorem and is mentioned and proved as part of the 31st proposition in the third book of Euclid's Elements. [1]

  7. Soddy circles of a triangle - Wikipedia

    en.wikipedia.org/wiki/Soddy_circles_of_a_triangle

    When the outer Soddy circle has positive curvature, both Soddy centers are equal detour points. When the outer Soddy circle has negative curvature, its center is the isoperimetric point: the triangles ABP 2, BCP 2, and CAP 2 have equal perimeter. In geometry, the Soddy circles of a triangle are two circles associated with any triangle in the

  8. Casey's theorem - Wikipedia

    en.wikipedia.org/wiki/Casey's_theorem

    The following proof is attributable [2] to Zacharias. [3] Denote the radius of circle by and its tangency point with the circle by . We will use the notation , for the centers of the circles. Note that from Pythagorean theorem,

  9. Power of a point - Wikipedia

    en.wikipedia.org/wiki/Power_of_a_point

    Due to the Pythagorean theorem the number () has the simple geometric meanings shown in the diagram: For a point outside the circle () is the squared tangential distance | | of point to the circle . Points with equal power, isolines of Π ( P ) {\displaystyle \Pi (P)} , are circles concentric to circle c {\displaystyle c} .