Editing Talk:1935: 2018
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| − | This is easy! Don't factor it - just multiply by 25 and if that ends in two zeros, but not four zeros then it's a leap year, at least most of the time.....17:25, 29 December 2017 (UTC) | + | This is easy! Don't factor it - just multiply by 25 and if that ends in two zeros, but not four zeros then it's a leap year, at least most of the time.....17:25, 29 December 2017 (UTC) |
This is easy! Don’t factor it - just convert it into a binary and look at the 2 least significant bits. If they are 00 the number is multiple of four. —[[Special:Contributions/172.69.33.35|172.69.33.35]] 17:37, 29 December 2017 (UTC) | This is easy! Don’t factor it - just convert it into a binary and look at the 2 least significant bits. If they are 00 the number is multiple of four. —[[Special:Contributions/172.69.33.35|172.69.33.35]] 17:37, 29 December 2017 (UTC) | ||
| − | This is easy! Don't factor it - just subtract 4 repeatedly. If you end up at 0, it's divisible. If you end up at 1, 2, or 3, it's not. -- 17:55, 29 December 2017 (UTC) | + | This is easy! Don't factor it - just subtract 4 repeatedly. If you end up at 0, it's divisible. If you end up at 1, 2, or 3, it's not. -- 17:55, 29 December 2017 (UTC) |
This ''is'' easy! Sums of numbers that have 4 as a factor are all divisible by four. (I'll leave the proof of that as an exercise for the reader, but it's really trivial, though possibly non-intuitive.) This means that one can take a number apart and check the individual pieces. Now, any number that's a multiple of 100 is divisible by four (10 * 10 = 5² * 2²,) so one can essentially cut away the higher digits of a number, as they do not influence its divisibility with regard to 4. Now look at the first of the remaining digits. If that's odd, add 2 to the last digit. If the last digit is now divisible by four, the original number is divisble by four. [[User:Tibfulv|Tibfulv]] ([[User talk:Tibfulv|talk]]) 00:38, 30 December 2017 (UTC) | This ''is'' easy! Sums of numbers that have 4 as a factor are all divisible by four. (I'll leave the proof of that as an exercise for the reader, but it's really trivial, though possibly non-intuitive.) This means that one can take a number apart and check the individual pieces. Now, any number that's a multiple of 100 is divisible by four (10 * 10 = 5² * 2²,) so one can essentially cut away the higher digits of a number, as they do not influence its divisibility with regard to 4. Now look at the first of the remaining digits. If that's odd, add 2 to the last digit. If the last digit is now divisible by four, the original number is divisble by four. [[User:Tibfulv|Tibfulv]] ([[User talk:Tibfulv|talk]]) 00:38, 30 December 2017 (UTC) | ||
