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  2. Midpoint polygon - Wikipedia

    en.wikipedia.org/wiki/Midpoint_polygon

    The midpoint polygon of a quadrilateral is a parallelogram called its Varignon parallelogram. If the quadrilateral is simple, the area of the parallelogram is one half the area of the original quadrilateral. The perimeter of the parallelogram equals the sum of the diagonals of the original quadrilateral.

  3. Missing square puzzle - Wikipedia

    en.wikipedia.org/wiki/Missing_square_puzzle

    Splitting the thin parallelogram area (yellow) into little parts, and building a single unit square with them The key to the puzzle is the fact that neither of the 13×5 "triangles" is truly a triangle, nor would either truly be 13x5 if it were, because what appears to be the hypotenuse is bent.

  4. Varignon's theorem - Wikipedia

    en.wikipedia.org/wiki/Varignon's_theorem

    An arbitrary quadrilateral and its diagonals. Bases of similar triangles are parallel to the blue diagonal. Ditto for the red diagonal. The base pairs form a parallelogram with half the area of the quadrilateral, A q, as the sum of the areas of the four large triangles, A l is 2 A q (each of the two pairs reconstructs the quadrilateral) while that of the small triangles, A s is a quarter of A ...

  5. Parallelogram - Wikipedia

    en.wikipedia.org/wiki/Parallelogram

    The perimeter of a parallelogram is 2(a + b) where a and b are the lengths of adjacent sides. Unlike any other convex polygon, a parallelogram cannot be inscribed in any triangle with less than twice its area. [7] The centers of four squares all constructed either internally or externally on the sides of a parallelogram are the vertices of a ...

  6. Rhomboid - Wikipedia

    en.wikipedia.org/wiki/Rhomboid

    Traditionally, in two-dimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are non-right angled.. The terms "rhomboid" and "parallelogram" are often erroneously conflated with each other (i.e, when most people refer to a "parallelogram" they almost always mean a rhomboid, a specific subtype of parallelogram); however, while all rhomboids ...

  7. Quadrilateral - Wikipedia

    en.wikipedia.org/wiki/Quadrilateral

    Rhomboid: a parallelogram in which adjacent sides are of unequal lengths, and some angles are oblique (equiv., having no right angles). Informally: "a pushed-over oblong". Not all references agree; some define a rhomboid as a parallelogram that is not a rhombus. [4] Rectangle: all four angles are right angles (equiangular). An equivalent ...

  8. Napkin folding problem - Wikipedia

    en.wikipedia.org/wiki/Napkin_folding_problem

    The napkin folding problem is a problem in geometry and the mathematics of paper folding that explores whether folding a square or a rectangular napkin can increase its perimeter. The problem is known under several names, including the Margulis napkin problem , suggesting it is due to Grigory Margulis , and the Arnold's rouble problem referring ...

  9. Area of a triangle - Wikipedia

    en.wikipedia.org/wiki/Area_of_a_triangle

    The area of a triangle can be demonstrated, for example by means of the congruence of triangles, as half of the area of a parallelogram that has the same base length and height. A graphic derivation of the formula T = h 2 b {\displaystyle T={\frac {h}{2}}b} that avoids the usual procedure of doubling the area of the triangle and then halving it.