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  2. List of formulae involving π - Wikipedia

    en.wikipedia.org/wiki/List_of_formulae_involving_π

    where A is the area of an epicycloid with the smaller circle of radius r and the larger circle of radius kr (), assuming the initial point lies on the larger circle. A = ( − 1 ) k + 3 8 π a 2 {\displaystyle A={\frac {(-1)^{k}+3}{8}}\pi a^{2}}

  3. Equation - Wikipedia

    en.wikipedia.org/wiki/Equation

    When R is chosen to have the value of 2 (R = 2), this equation would be recognized in Cartesian coordinates as the equation for the circle of radius of 2 around the origin. Hence, the equation with R unspecified is the general equation for the circle. Usually, the unknowns are denoted by letters at the end of the alphabet, x, y, z, w ...

  4. Circle - Wikipedia

    en.wikipedia.org/wiki/Circle

    The sum of the squared lengths of any two chords intersecting at right angles at a given point is the same as that of any other two perpendicular chords intersecting at the same point and is given by 8r 2 − 4p 2, where r is the circle radius, and p is the distance from the centre point to the point of intersection. [14]

  5. Spherical coordinate system - Wikipedia

    en.wikipedia.org/wiki/Spherical_coordinate_system

    For example, one sphere that is described in Cartesian coordinates with the equation x 2 + y 2 + z 2 = c 2 can be described in spherical coordinates by the simple equation r = c. (In this system—shown here in the mathematics convention—the sphere is adapted as a unit sphere, where the radius is set to unity and then can generally be ignored ...

  6. Descartes' theorem - Wikipedia

    en.wikipedia.org/wiki/Descartes'_theorem

    By solving this equation, one can construct a fourth circle tangent to three given, mutually tangent circles. The theorem is named after René Descartes, who stated it in 1643. Frederick Soddy's 1936 poem The Kiss Precise summarizes the theorem in terms of the bends (signed inverse radii) of the four circles:

  7. Area of a circle - Wikipedia

    en.wikipedia.org/wiki/Area_of_a_circle

    Another proof that uses triangles considers the area enclosed by a circle to be made up of an infinite number of triangles (i.e. the triangles each have an angle of dπœƒ at the centre of the circle), each with an area of ⁠ 1 / 2 ⁠ · r 2 · dπœƒ (derived from the expression for the area of a triangle: ⁠ 1 / 2 ⁠ · a · b · sinπœƒ ...

  8. Gauss circle problem - Wikipedia

    en.wikipedia.org/wiki/Gauss_circle_problem

    Consider a circle in with center at the origin and radius . Gauss's circle problem asks how many points there are inside this circle of the form ( m , n ) {\displaystyle (m,n)} where m {\displaystyle m} and n {\displaystyle n} are both integers.

  9. Radial function - Wikipedia

    en.wikipedia.org/wiki/Radial_function

    For example, a radial function Φ in two dimensions has the form [1] (,) = (), = + where φ is a function of a single non-negative real variable. Radial functions are contrasted with spherical functions , and any descent function (e.g., continuous and rapidly decreasing ) on Euclidean space can be decomposed into a series consisting of radial ...