How to Subtract By Adding
Brought to you by http://www.lynda.com/minutephysics https://en.wikipedia.org/wiki/Method_of_complements Hank's adding machine video: https://www.youtube.com/watch?v=JYT8oH-NODs Thanks to...
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Suppose you want to know how long it was between the Norman invasion of England and Columbus'sarrival in the Caribbean; that is, what’s 1492 minus 1066 – but you hate long subtractionwith its borrowing and such.Well, we can subtract by adding: first we just need to replace each digit of the smallernumber with 9 minus that digit, except the final digit gets replaced with 10 minus thatdigit, so 1066 becomes nine minus 1, nine minus 0, nine minus six, ten minus 6. Addingthat to 1492 gives 10426, and if we ignore the first digit, we get the answer: 1492 minus1066 is 426. You can check just to be sure. Coincidentally, it’s roughly 426 times fartherfrom Portugal to the West Indies and back than from France to England.But anyway, the subtraction-by-adding trick wor...
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Suppose you want to know how long it was between the Norman invasion of England and Columbus'sarrival in the Caribbean; that is, what’s 1492 minus 1066 – but you hate long subtractionwith its borrowing and such.Well, we can subtract by adding: first we just need to replace each digit of the smallernumber with 9 minus that digit, except the final digit gets replaced with 10 minus thatdigit, so 1066 becomes nine minus 1, nine minus 0, nine minus six, ten minus 6. Addingthat to 1492 gives 10426, and if we ignore the first digit, we get the answer: 1492 minus1066 is 426. You can check just to be sure. Coincidentally, it’s roughly 426 times fartherfrom Portugal to the West Indies and back than from France to England.But anyway, the subtraction-by-adding trick wor...
