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  2. Ptolemy's theorem - Wikipedia

    en.wikipedia.org/wiki/Ptolemy's_theorem

    Any square can be inscribed in a circle whose center is the center of the square. If the common length of its four sides is equal to a {\displaystyle a} then the length of the diagonal is equal to a 2 {\displaystyle a{\sqrt {2}}} according to the Pythagorean theorem , and Ptolemy's relation obviously holds.

  3. Squaring the circle - Wikipedia

    en.wikipedia.org/wiki/Squaring_the_circle

    Squaring the circle is a problem in geometry first proposed in Greek mathematics.It is the challenge of constructing a square with the area of a given circle by using only a finite number of steps with a compass and straightedge.

  4. Area of a circle - Wikipedia

    en.wikipedia.org/wiki/Area_of_a_circle

    Circle with square and octagon inscribed, showing area gap. Suppose that the area C enclosed by the circle is greater than the area T = cr/2 of the triangle. Let E denote the excess amount. Inscribe a square in the circle, so that its four corners lie on the circle. Between the square and the circle are four segments.

  5. Square - Wikipedia

    en.wikipedia.org/wiki/Square

    The external angle of a square is equal to 90°. The diagonals of a square are equal and bisect each other, meeting at 90°. The diagonal of a square bisects its internal angle, forming adjacent angles of 45°. All four sides of a square are equal. Opposite sides of a square are parallel. A square has Schläfli symbol {4}.

  6. Thales's theorem - Wikipedia

    en.wikipedia.org/wiki/Thales's_theorem

    the square of the length of a vector is given by the dot product of the vector with itself. Let there be a right angle ∠ ABC and circle M with AC as a diameter. Let M's center lie on the origin, for easier calculation. Then we know A = −C, because the circle centered at the origin has AC as diameter, and

  7. Orthodiagonal quadrilateral - Wikipedia

    en.wikipedia.org/wiki/Orthodiagonal_quadrilateral

    The area K of an orthodiagonal quadrilateral equals one half the product of the lengths of the diagonals p and q: [8] =. Conversely, any convex quadrilateral where the area can be calculated with this formula must be orthodiagonal. [6]

  8. Golden rectangle - Wikipedia

    en.wikipedia.org/wiki/Golden_rectangle

    Owing to the Pythagorean theorem, the diagonal dividing one half of a square equals the radius of a circle whose outermost point is the corner of a golden rectangle added to the square. [1] Thus, a golden rectangle can be constructed with only a straightedge and compass in four steps: Draw a square

  9. Brahmagupta's formula - Wikipedia

    en.wikipedia.org/wiki/Brahmagupta's_formula

    This formula generalizes Heron's formula for the area of a triangle. A triangle may be regarded as a quadrilateral with one side of length zero. From this perspective, as d approaches zero, a cyclic quadrilateral converges into a cyclic triangle (all triangles are cyclic), and Brahmagupta's formula simplifies to Heron's formula.