British TV presenter and popularizer of mathematics Johnny Ball in the new issue of Numberphile talked about the old "Russian" way of multiplying numbers. According to him, this unusual method was used in Russia in the 19th century, although it was known in Ancient Egypt.
This method involves stepwise division by two of the first number and multiplication by two of the second. Johnny Ball explained everything with the example of works 9 and 13 (video can be watched from 0:40).
On the left, 9 is divided by 2, and on the right, 13 is multiplied by 2. The results of the operations were recorded under the original numbers until one remained on the left. Fractional parts were discarded during division. As a result, we got two columns with numbers: 9, 4, 2, 1 and 13, 26, 52, 104. Next, you need to cross out the lines where even numbers are obtained on the left. In this case, these are 4 and 26, 2 and 52. After that, it remains only to add the numbers remaining on the right: 13 and 104 - their sum will be equal to the product of the original 9 and 13 - this is 117.
This method seems too complicated for such primes, but it can be very useful when multiplying two-digit and three-digit numbers, as Ball demonstrated later in the video.
Did you know about this method? Let us know in the comments.
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