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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. Circle - Wikipedia

    en.wikipedia.org/wiki/Circle

    A circle bounds a region of the plane called a disc. The circle has been known since before the beginning of recorded history. Natural circles are common, such as the full moon or a slice of round fruit. The circle is the basis for the wheel, which, with related inventions such as gears, makes much of modern

  4. Intersecting chords theorem - Wikipedia

    en.wikipedia.org/wiki/Intersecting_chords_theorem

    In Euclidean geometry, the intersecting chords theorem, or just the chord theorem, is a statement that describes a relation of the four line segments created by two intersecting chords within a circle. It states that the products of the lengths of the line segments on each chord are equal. It is Proposition 35 of Book 3 of Euclid's Elements.

  5. Nine-point center - Wikipedia

    en.wikipedia.org/wiki/Nine-point_center

    A triangle showing its circumcircle and circumcenter (black), altitudes and orthocenter (red), and nine-point circle and nine-point center (blue) In geometry , the nine-point center is a triangle center , a point defined from a given triangle in a way that does not depend on the placement or scale of the triangle.

  6. Area of a circle - Wikipedia

    en.wikipedia.org/wiki/Area_of_a_circle

    The transformation sends the circle to an ellipse by stretching or shrinking the horizontal and vertical diameters to the major and minor axes of the ellipse. The square gets sent to a rectangle circumscribing the ellipse. The ratio of the area of the circle to the square is π /4, which means the ratio of the ellipse to the rectangle is also π /4

  7. Conic section - Wikipedia

    en.wikipedia.org/wiki/Conic_section

    A conic is the curve obtained as the intersection of a plane, called the cutting plane, with the surface of a double cone (a cone with two nappes).It is usually assumed that the cone is a right circular cone for the purpose of easy description, but this is not required; any double cone with some circular cross-section will suffice.

  8. Inversive geometry - Wikipedia

    en.wikipedia.org/wiki/Inversive_geometry

    Circles G to J, which do not, map to other circles. The reference circle and line L map to themselves. Circles intersect their inverses, if any, on the reference circle. In the SVG file, click or hover over a circle to highlight it. Inversion of a line is a circle containing the center of inversion; or it is the line itself if it contains the ...

  9. Pole and polar - Wikipedia

    en.wikipedia.org/wiki/Pole_and_polar

    The pole of a line L in a circle C is a point Q that is the inversion in C of the point P on L that is closest to the center of the circle. Conversely, the polar line (or polar) of a point Q in a circle C is the line L such that its closest point P to the center of the circle is the inversion of Q in C.