The method of casting out nines has long been used as a check for manual arithmetic calculations. Casting out 9s is done by adding the digits of a number, and then repeating this process until a single digit number is reached. Through this process we can check the validity of our computations. We'll discuss how casting out 9s involves something called modular congruence, which concerns the remainder after a division - and from here we see how we can cast out 99s, and cast out 999s to test for certain divisibilities - namely, 37. #mathtricks #maths #mathematics
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0:00 Intro
1:36 Casting out 9s
5:57 Why Casting out 9s Works
9:52 Another Example
10:34 Modular Congruence
11:30 Merely a Check
13:39 Division
16:05 Remainders with 3
17:32 Casting out 11s
19:43 Divisibility Rule for 37
23:38 Conclusion