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Since 9 = 10 − 1, to multiply a number by nine, multiply it by 10 and then subtract the original number from the result. For example, 9 × 27 = 270 − 27 = 243. This method can be adjusted to multiply by eight instead of nine, by doubling the number being subtracted; 8 × 27 = 270 − (2×27) = 270 − 54 = 216.
The Karatsuba algorithm is a fast multiplication algorithm. It was discovered by Anatoly Karatsuba in 1960 and published in 1962. [ 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 n log 2 3 ...
Subtract the rightmost digit from 10. Subtract the remaining digits from 9. Double the result. Add half of the neighbor to the right, plus 5 if the digit is odd. For the leading zero, subtract 2 from half of the neighbor. Example: 492 × 3 = 1476 Working from right to left: (10 − 2) × 2 + Half of 0 (0) = 16. Write 6, carry 1.
If a positional numeral system is used, a natural way of multiplying numbers is taught in schools as long multiplication, sometimes called grade-school multiplication, sometimes called the Standard Algorithm: multiply the multiplicand by each digit of the multiplier and then add up all the properly shifted results.
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Byster used to work as a floor trader at the Chicago Mercantile Exchange, but after his cousin, a math teacher in a Chicago area high school, invited him to show the class his shortcuts for doing base 10 arithmetic, Byster quit his job to devote himself to teaching children his methods. [3]
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