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I wonder if it would help to remember that etymologically "eleven" comes from "one left" (as in, I counted the first ten and there was still one more) and "twel
by jacobolus 2y ago
I wonder if it would help to remember that etymologically "eleven" comes from "one left" (as in, I counted the first ten and there was still one more) and "twelve" comes from "two left".
As for the others, I think remembering these in several different ways is stickiest. For example, you might think of 7·7 = (5 + 2)² = 25 + 2·10 + 4 or perhaps 7·7 = (6 + 1)² = 36 + 2·6 + 1 or 7·7 = 7·(10 − 3) = 70 − 21. If you already know 7·7, then 6·8 = (7 − 1)(7 + 1) = 49 − 1. You can try computing 7·8 by repeatedly doubling: 7, 14, 28, 56. Etc.
- DataDaoDe 2y agoI like the shove it to the nearest 10 approach. It makes a lot of calculations much simpler b/c they can be transformed to a simple multiplication by 10 and a addition or subtraction or two. 1. 6⋅7 = (6⋅10) - (6⋅3) = 60 - 18 = 42 2. 7⋅7 = (7⋅10) - (7⋅3) = 70 - 21 = 49 3. 13⋅19 = (13⋅20) - (13⋅1) = 260 - 13 = 247 4. 58⋅61 = (58⋅60) + (58⋅1) = 3480 + 58 = 3538 If we go up another order of magnitude, then the system starts really grinding to a halt though tbh :)
- card_zero 2y agoThe squares of primes become memorable if you've ever tried searching for primes in your head. That's because 7*7 is the smallest product of prime factors that are all larger than or equal to 7: in other words, you can check for the primality of numbers smaller than that by testing for division by 2, 3, or 5 only, because they must divide by one of those or be prime. Because of this pointless mental exercise it also sticks in my mind that 11 squared is 121 and 13 squared is 169 (though the presence of 69 helps with that one).