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  2. Proof by contradiction - Wikipedia

    en.wikipedia.org/wiki/Proof_by_contradiction

    An early occurrence of proof by contradiction can be found in Euclid's Elements, Book 1, Proposition 6: [7] If in a triangle two angles equal one another, then the sides opposite the equal angles also equal one another. The proof proceeds by assuming that the opposite sides are not equal, and derives a contradiction.

  3. A Mathematician's Lament - Wikipedia

    en.wikipedia.org/wiki/A_Mathematician's_Lament

    978-1-934137-17-8 A Mathematician's Lament , often referred to informally as Lockhart's Lament , is a short book on mathematics education by Paul Lockhart, originally a research mathematician at Brown University and U.C. Santa Cruz, and subsequently a math teacher at Saint Ann's School in Brooklyn , New York City for many years.

  4. Quizlet - Wikipedia

    en.wikipedia.org/wiki/Quizlet

    Quizlet's primary products include digital flash cards, matching games, practice electronic assessments, and live quizzes. In 2017, 1 in 2 high school students used Quizlet. [4] As of December 2021, Quizlet has over 500 million user-generated flashcard sets and more than 60 million active users. [5]

  5. Opposite group - Wikipedia

    en.wikipedia.org/wiki/Opposite_group

    g 1, g 2 denotes the ordered pair of the two group elements. *' can be viewed as the naturally induced addition of +. In group theory , a branch of mathematics , an opposite group is a way to construct a group from another group that allows one to define right action as a special case of left action .

  6. Law of noncontradiction - Wikipedia

    en.wikipedia.org/wiki/Law_of_noncontradiction

    In logic, the law of non-contradiction (LNC; also known as the law of contradiction, principle of non-contradiction (PNC), or the principle of contradiction) states that propositions cannot both be true and false at the same time, e. g. the two propositions "the house is white" and "the house is not white" are mutually exclusive.

  7. Crossed ladders problem - Wikipedia

    en.wikipedia.org/wiki/Crossed_ladders_problem

    1 / 1/2 ⁠ + ⁠ 1 / 1 ⁠ = ⁠ 1 / h ⁠ ∴ 2 + 1 = ⁠ 1 / h ⁠ ∴ h = ⁠ 1 / 2 + 1 ⁠ = ⁠ 1 / 3 ⁠ One side (left in the illustration) is partially folded in half and pinched to leave a mark. The intersection of a line from this mark to an opposite corner (red) with a diagonal (blue) is exactly one third from the bottom edge.

  8. Hom functor - Wikipedia

    en.wikipedia.org/wiki/Hom_functor

    The functor Hom(–, B) is also called the functor of points of the object B. Note that fixing the first argument of Hom naturally gives rise to a covariant functor and fixing the second argument naturally gives a contravariant functor.

  9. Dual (category theory) - Wikipedia

    en.wikipedia.org/wiki/Dual_(category_theory)

    In category theory, a branch of mathematics, duality is a correspondence between the properties of a category C and the dual properties of the opposite category C op.Given a statement regarding the category C, by interchanging the source and target of each morphism as well as interchanging the order of composing two morphisms, a corresponding dual statement is obtained regarding the opposite ...