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  2. Parallelogram law - Wikipedia

    en.wikipedia.org/wiki/Parallelogram_law

    In mathematics, the simplest form of the parallelogram law (also called the parallelogram identity) belongs to elementary geometry. It states that the sum of the squares of the lengths of the four sides of a parallelogram equals the sum of the squares of the lengths of the two diagonals. We use these notations for the sides: AB, BC, CD, DA.

  3. Varignon's theorem - Wikipedia

    en.wikipedia.org/wiki/Varignon's_theorem

    From the first proof, one can see that the sum of the diagonals is equal to the perimeter of the parallelogram formed. Also, we can use vectors 1/2 the length of each side to first determine the area of the quadrilateral, and then to find areas of the four triangles divided by each side of the inner parallelogram.

  4. Parallelogram - Wikipedia

    en.wikipedia.org/wiki/Parallelogram

    In Euclidean geometry, a parallelogram is a simple (non- self-intersecting) quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of equal measure. The congruence of opposite sides and opposite angles is a direct consequence of the ...

  5. Pythagorean theorem - Wikipedia

    en.wikipedia.org/wiki/Pythagorean_theorem

    In mathematics, the Pythagorean theorem or Pythagoras' theorem is a fundamental relation in Euclidean geometry between the three sides of a right triangle.It states that the area of the square whose side is the hypotenuse (the side opposite the right angle) is equal to the sum of the areas of the squares on the other two sides.

  6. Pappus's area theorem - Wikipedia

    en.wikipedia.org/wiki/Pappus's_area_theorem

    dark grey area = light grey area. Pappus's area theorem describes the relationship between the areas of three parallelograms attached to three sides of an arbitrary triangle. The theorem, which can also be thought of as a generalization of the Pythagorean theorem, is named after the Greek mathematician Pappus of Alexandria (4th century AD), who ...

  7. Apollonius's theorem - Wikipedia

    en.wikipedia.org/wiki/Apollonius's_theorem

    Proof of Apollonius's theorem. The theorem can be proved as a special case of Stewart's theorem, or can be proved using vectors (see parallelogram law). The following is an independent proof using the law of cosines. [1] Let the triangle have sides ,, with a median drawn to side .

  8. Theorem of the gnomon - Wikipedia

    en.wikipedia.org/wiki/Theorem_of_the_gnomon

    The theorem of the gnomon can be used to construct a new parallelogram or rectangle of equal area to a given parallelogram or rectangle by the means of straightedge and compass constructions. This also allows the representation of a division of two numbers in geometrical terms, an important feature to reformulate geometrical problems in ...

  9. Area of a triangle - Wikipedia

    en.wikipedia.org/wiki/Area_of_a_triangle

    In geometry, calculating the area of a triangle is an elementary problem encountered often in many different situations. The best known and simplest formula is where b is the length of the base of the triangle, and h is the height or altitude of the triangle. The term "base" denotes any side, and "height" denotes the length of a perpendicular ...