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  2. Tangent lines to circles - Wikipedia

    en.wikipedia.org/wiki/Tangent_lines_to_circles

    For two circles, there are generally four distinct lines that are tangent to both – if the two circles are outside each other – but in degenerate cases there may be any number between zero and four bitangent lines; these are addressed below. For two of these, the external tangent lines, the circles fall on the same side of the line; for the ...

  3. Problem of Apollonius - Wikipedia

    en.wikipedia.org/wiki/Problem_of_Apollonius

    To determine the shape of Z D, fix two distinct circles C 0 and C ∞, not necessarily tangent to D. These two circles determine a pencil, meaning a line L in the P 3 of circles. If the equations of C 0 and C ∞ are f and g, respectively, then the points on L correspond to the circles whose equations are Sf + Tg, where [S : T] is a point of P 1.

  4. Tangent circles - Wikipedia

    en.wikipedia.org/wiki/Tangent_circles

    In geometry, tangent circles (also known as kissing circles) are circles in a common plane that intersect in a single point. There are two types of tangency : internal and external. Many problems and constructions in geometry are related to tangent circles; such problems often have real-life applications such as trilateration and maximizing the ...

  5. Tangent - Wikipedia

    en.wikipedia.org/wiki/Tangent

    Two distinct circles lying in the same plane are said to be tangent to each other if they meet at exactly one point. If points in the plane are described using Cartesian coordinates , then two circles , with radii r 1 , r 2 {\displaystyle r_{1},r_{2}} and centers ( x 1 , y 1 ) {\displaystyle (x_{1},y_{1})} and ( x 2 , y 2 ) {\displaystyle (x_{2 ...

  6. Descartes' theorem - Wikipedia

    en.wikipedia.org/wiki/Descartes'_theorem

    The theorem does not apply to systems of circles with more than two circles tangent to each other at the same point. It requires that the points of tangency be distinct. [8] When more than two circles are tangent at a single point, there can be infinitely many such circles, with arbitrary curvatures; see pencil of circles. [27]

  7. Monge's theorem - Wikipedia

    en.wikipedia.org/wiki/Monge's_theorem

    For any two circles in a plane, an external tangent is a line that is tangent to both circles but does not pass between them. There are two such external tangent lines for any two circles. Each such pair has a unique intersection point in the extended Euclidean plane. Monge's theorem states that the three such points given by the three pairs of ...

  8. Circle - Wikipedia

    en.wikipedia.org/wiki/Circle

    Two tangents can always be drawn to a circle from any point outside the circle, and these tangents are equal in length. If a tangent at A and a tangent at B intersect at the exterior point P , then denoting the centre as O , the angles ∠ BOA and ∠ BPA are supplementary.

  9. Inversive geometry - Wikipedia

    en.wikipedia.org/wiki/Inversive_geometry

    A circle (or line) is unchanged by inversion if and only if it is orthogonal to the reference circle at the points of intersection. [5] Additional properties include: If a circle q passes through two distinct points A and A' which are inverses with respect to a circle k, then the circles k and q are orthogonal.