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  2. Triangular number - Wikipedia

    en.wikipedia.org/wiki/Triangular_number

    A triangular number or triangle number counts objects arranged in an equilateral triangle. Triangular numbers are a type of figurate number , other examples being square numbers and cube numbers . The n th triangular number is the number of dots in the triangular arrangement with n dots on each side, and is equal to the sum of the n natural ...

  3. Missing square puzzle - Wikipedia

    en.wikipedia.org/wiki/Missing_square_puzzle

    The apparent triangles formed from the figures are 13 units wide and 5 units tall, so it appears that the area should be S = ⁠ 13×5 / 2 ⁠ = 32.5 units. However, the blue triangle has a ratio of 5:2 (=2.5), while the red triangle has the ratio 8:3 (≈2.667), so the apparent combined hypotenuse in each figure is actually bent.

  4. Knuckle mnemonic - Wikipedia

    en.wikipedia.org/wiki/Knuckle_mnemonic

    One form of the mnemonic is done by counting on the knuckles of one's hand to remember the number of days in each month. [1] Knuckles are counted as 31 days, depressions between knuckles as 30 (or 28/29) days.

  5. Triangular array - Wikipedia

    en.wikipedia.org/wiki/Triangular_array

    Narayana triangle, counting strings of balanced parentheses with a given number of distinct nestings [7] Pascal's triangle , whose entries are the binomial coefficients [ 8 ] Triangular arrays of integers in which each row is symmetric and begins and ends with 1 are sometimes called generalized Pascal triangles ; examples include Pascal's ...

  6. Magic triangle (mathematics) - Wikipedia

    en.wikipedia.org/wiki/Magic_triangle_(mathematics)

    A magic triangle or perimeter magic triangle [1] is an arrangement of the integers from 1 to n on the sides of a triangle with the same number of integers on each side, called the order of the triangle, so that the sum of integers on each side is a constant, the magic sum of the triangle.

  7. Twenty-One Card Trick - Wikipedia

    en.wikipedia.org/wiki/Twenty-One_Card_Trick

    Mathematical explanation of the Twenty-one card trick with 27 cards: In each step, the cards are dealt into three piles. The piles are accumulated with the pile containing the target card (shaded yellow and labelled with the step number) put in the middle. After three steps, the middle card (*) is the one in all chosen piles.

  8. Cuisenaire rods - Wikipedia

    en.wikipedia.org/wiki/Cuisenaire_rods

    Cuisenaire rods illustrating the factors of ten A demonstration the first pair of amicable numbers, (220,284). Cuisenaire rods are mathematics learning aids for pupils that provide an interactive, hands-on [1] way to explore mathematics and learn mathematical concepts, such as the four basic arithmetical operations, working with fractions and finding divisors.

  9. Tally marks - Wikipedia

    en.wikipedia.org/wiki/Tally_marks

    Tally marks, also called hash marks, are a form of numeral used for counting. They can be thought of as a unary numeral system. They are most useful in counting or tallying ongoing results, such as the score in a game or sport, as no intermediate results need to be erased or discarded. However, because of the length of large numbers, tallies ...