- Veda #1 : All from 9 and last from 10
- dividing by a factor of 100 is the same as multiplying by 100/factor and moving the decimal
- 9/11 times a number is 10 times the number, minus/plus the number
- 6/4 times a number is half the number times ten, plus/minus the number
- when multiplying two numbers, take midpoint square and subtract difference as a square (e.g., 50x54 52x52-2^2) - “difference of squares”
- to multiply by 7, take the number being multiplied by 10 and subtract 3 times the same number
- square large numbers using binomial expansion - (n+a)^2= n^2 + 2na + a^2
- difference between consecutive squares is 2x - 1 or 2x + 1 where x is next closest square depending on going up or down
- dividing by 9 is multiplying by .1111 so 8/9 = .8888 or 888/9 = 888+88+8+.8+.88... etc then move the decimal
- when subtracting, for the 1's digit, if the left number is bigger than the right, it's just the subtraction - but if the left number is smaller than the right it's the subtraction plus 10.
- example : 23-21 = 2
- example 2 : 21 - 13 = (1-3)+10 = 8
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