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The grid method (also known as the box method) of multiplication is an introductory approach to multi-digit multiplication calculations that involve numbers larger than ten. Because it is often taught in mathematics education at the level of primary school or elementary school , this algorithm is sometimes called the grammar school method.
To multiply two numbers with n digits using this method, one needs about n 2 operations. More formally, multiplying two n-digit numbers using long multiplication requires Θ(n 2) single-digit operations (additions and multiplications).
[1] [2] [3] It is a divide-and-conquer algorithm that reduces the multiplication of two n-digit numbers to three multiplications of n/2-digit numbers and, by repeating this reduction, to at most single-digit multiplications.
Trachtenberg defined this algorithm with a kind of pairwise multiplication where two digits are multiplied by one digit, essentially only keeping the middle digit of the result. By performing the above algorithm with this pairwise multiplication, even fewer temporary results need to be held. Example:
Four bags with three marbles per bag gives twelve marbles (4 × 3 = 12). Multiplication can also be thought of as scaling. Here, 2 is being multiplied by 3 using scaling, giving 6 as a result. Animation for the multiplication 2 × 3 = 6 4 × 5 = 20. The large rectangle is made up of 20 squares, each 1 unit by 1 unit.
Therefore each hand can represent the digits 0-9, rather than the usual 0-5. The two hands combine to represent two digits; the right hand is the ones place, and the left hand is the tens place. This way, any number from 0 to 99 can be shown, and it's possible to count up to 99 instead of just 10.
Multiplication 4: 14 "You Ain't Nothing but a Houndred" Multiplication of 3-digit numbers by single digits 5: 15 "Dividing we Stand" Dividing 3-digit numbers by 2-digit numbers 6: 16 "Double Digit Dating" Multiplying double digit numbers 7: 17 "Six Flat Worms" Area 8: 18 "Learn your Lines" Parallel and Perpendicular lines 9: 19 "Rectangle Delight"
As an example, consider the multiplication of 58 with 213. After writing the multiplicands on the sides, consider each cell, beginning with the top left cell. In this case, the column digit is 5 and the row digit is 2. Write their product, 10, in the cell, with the digit 1 above the diagonal and the digit 0 below the diagonal (see picture for ...
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