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These equations express that the tangent line, which is parallel to , is perpendicular to the radii, and that the tangent points lie on their respective circles. These are four quadratic equations in two two-dimensional vector variables, and in general position will have four pairs of solutions.
Kissing circles. Given three mutually tangent circles (black), there are, in general, two possible answers (red) as to what radius a fourth tangent circle can have.In geometry, Descartes' theorem states that for every four kissing, or mutually tangent, circles, the radii of the circles satisfy a certain quadratic equation.
When the centre of the circle is at the origin, then the equation of the tangent line becomes + =, and its slope is =. Properties The circle is the shape with the largest area for a given length of perimeter (see Isoperimetric inequality ).
Tangent lines to circles; Circle packing theorem, the result that every planar graph may be realized by a system of tangent circles; Hexafoil, the shape formed by a ring of six tangent circles; Feuerbach's theorem on the tangency of the nine-point circle of a triangle with its incircle and excircles; Descartes' theorem; Ford circle; Bankoff circle
Constructing a tangent using Thales's theorem. Thales's theorem can be used to construct the tangent to a given circle that passes through a given point. In the figure at right, given circle k with centre O and the point P outside k, bisect OP at H and draw the circle of radius OH with centre H.
The equation of the tangent line at a point (X,Y) such that f(X,Y) = 0 is then [12] ... Two pairs of tangent circles. Above internally and below externally tangent.
All tangent circles to the given circles can be found by varying line . Positions of the centers Circles tangent to two circles. If is the center and the radius of the circle, that is tangent to the given circles at the points ,, then:
Illustration of the sine and tangent inequalities. The figure at the right shows a sector of a circle with radius 1. The sector is θ/(2 π) of the whole circle, so its area is θ/2. We assume here that θ < π /2. = = = =