Comments on: Simple tricks for doing arithmetic in your head http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/ tech, science, news and social issues for geeks Tue, 14 Feb 2012 07:53:04 +0000 hourly 1 http://wordpress.org/?v= By: Narenhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-306021 Naren Fri, 22 Jul 2011 03:44:34 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-306021 Anu Are you the same anu who did a course in AMES training centre back in 2000 in Auckland?.IF you are then reply back - naren_2010@hotmail.com Anu

Are you the same anu who did a course in AMES training centre back in 2000 in Auckland?.IF you are then reply back – naren_2010@hotmail.com

]]> By: Sterling Camdenhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256983 Sterling Camden Sun, 11 Jul 2010 17:53:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256983 That's fascinating, Ari. That's fascinating, Ari.

]]> By: Sterling Camdenhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276240 Sterling Camden Sun, 11 Jul 2010 17:53:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276240 That's fascinating, Ari. That’s fascinating, Ari.

]]> By: Ari D Jordonhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256977 Ari D Jordon Sun, 11 Jul 2010 17:20:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256977 Actually, there really are only three divisibility tricks. Interestingly, they work in any base, not just base 10. For a given base B: 1) n B - 1: Take the last digit, multiply by n, and add it to remaining digits. Repeat until you can tell whether the number is divisible by n B - 1 2) B^n: If the last n digits are divisible by B^n, so is the number 3) n B + 1: Take the last digit, multiply by n, and subtract it from the remaining digits. Repeat until you can tell In each case, if the relevant number isn't prime, the rule also applies to all of its factors. For base 10: 2, 4, 5, 8 are all cases of rule 2. n=1 for 2 and 5, 2 for 4, 3 for 8 3 and 9 are cases of rule 1 with n=1 7 and 11 (and 3) are cases of rule 3. For 11, n=1. For 7, n=2 (2 * 10 + 1 = 21, factors are 3 and 7) Actually, there really are only three divisibility tricks. Interestingly, they work in any base, not just base 10.

For a given base B:

1) n B – 1: Take the last digit, multiply by n, and add it to remaining digits. Repeat until you can tell whether the number is divisible by n B – 1

2) B^n: If the last n digits are divisible by B^n, so is the number

3) n B + 1: Take the last digit, multiply by n, and subtract it from the remaining digits. Repeat until you can tell

In each case, if the relevant number isn't prime, the rule also applies to all of its factors. For base 10:

2, 4, 5, 8 are all cases of rule 2. n=1 for 2 and 5, 2 for 4, 3 for 8

3 and 9 are cases of rule 1 with n=1

7 and 11 (and 3) are cases of rule 3. For 11, n=1. For 7, n=2 (2 * 10 + 1 = 21, factors are 3 and 7)

]]> By: Ari D Jordonhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276239 Ari D Jordon Sun, 11 Jul 2010 17:20:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276239 Actually, there really are only three divisibility tricks. Interestingly, they work in any base, not just base 10.For a given base B:1) n B - 1: Take the last digit, multiply by n, and add it to remaining digits. Repeat until you can tell whether the number is divisible by n B - 1 2) B^n: If the last n digits are divisible by B^n, so is the number 3) n B + 1: Take the last digit, multiply by n, and subtract it from the remaining digits. Repeat until you can tellIn each case, if the relevant number isn't prime, the rule also applies to all of its factors. For base 10:2, 4, 5, 8 are all cases of rule 2. n=1 for 2 and 5, 2 for 4, 3 for 8 3 and 9 are cases of rule 1 with n=1 7 and 11 (and 3) are cases of rule 3. For 11, n=1. For 7, n=2 (2 * 10 + 1 = 21, factors are 3 and 7) Actually, there really are only three divisibility tricks. Interestingly, they work in any base, not just base 10.

For a given base B:

1) n B – 1: Take the last digit, multiply by n, and add it to remaining digits. Repeat until you can tell whether the number is divisible by n B – 1
2) B^n: If the last n digits are divisible by B^n, so is the number
3) n B + 1: Take the last digit, multiply by n, and subtract it from the remaining digits. Repeat until you can tell

In each case, if the relevant number isn’t prime, the rule also applies to all of its factors. For base 10:

2, 4, 5, 8 are all cases of rule 2. n=1 for 2 and 5, 2 for 4, 3 for 8
3 and 9 are cases of rule 1 with n=1
7 and 11 (and 3) are cases of rule 3. For 11, n=1. For 7, n=2 (2 * 10 + 1 = 21, factors are 3 and 7)

]]> By: Sean Crosserhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256971 Sean Crosser Sun, 11 Jul 2010 16:21:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256971 Sorry, it turned out different than I typed in. It uses a variant of the first one, but it's actually able to do it with more that just x11. 35 x 7 = 245, and 35 x 60 = 2100, and 2100 + 245 = 2345. Mayb a big number isn't practical for an example, but 25 x 15, or smaller number with lower digits. Sorry, it turned out different than I typed in. It uses a variant of the first one, but it's actually able to do it with more that just x11.

35 x 7 = 245, and 35 x 60 = 2100, and 2100 + 245 = 2345. Mayb a big number isn't practical for an example, but 25 x 15, or smaller number with lower digits.

]]> By: Sean Crosserhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276238 Sean Crosser Sun, 11 Jul 2010 16:21:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276238 Sorry, it turned out different than I typed in. It uses a variant of the first one, but it's actually able to do it with more that just x11.35 x 7 = 245, and 35 x 60 = 2100, and 2100 + 245 = 2345. Mayb a big number isn't practical for an example, but 25 x 15, or smaller number with lower digits. Sorry, it turned out different than I typed in. It uses a variant of the first one, but it’s actually able to do it with more that just x11.

35 x 7 = 245, and 35 x 60 = 2100, and 2100 + 245 = 2345. Mayb a big number isn’t practical for an example, but 25 x 15, or smaller number with lower digits.

]]> By: Sean Crosserhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256970 Sean Crosser Sun, 11 Jul 2010 16:15:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-256970 35 x 67 245 210 2345 35

x 67

245

210

2345

]]> By: Sean Crosserhttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276237 Sean Crosser Sun, 11 Jul 2010 16:15:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276237 35 x 67 245 210 2345 35
x 67
245
210
2345

]]> By: Anu haridashttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-252291 Anu haridas Mon, 10 May 2010 05:45:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-252291 Most of them know it b4:)try t includ someth new..thankyou Most of them know it b4:)try t includ someth new..thankyou

]]> By: Anu haridashttp://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276236 Anu haridas Mon, 10 May 2010 05:45:00 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-276236 Most of them know it b4:)try t includ someth new..thankyou Most of them know it b4:)try t includ someth new..thankyou

]]> By: Whimsy Girl » Blog Archive » find if a number is a multiple of (insert#)http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-132680 Whimsy Girl » Blog Archive » find if a number is a multiple of (insert#) Sat, 31 Jan 2009 19:46:35 +0000 http://www.geeksaresexy.net/2008/02/19/simple-tricks-for-doing-arithmetic-in-your-head/#comment-132680 [...] Credit goes to GEEKSARESEXY [...] [...] Credit goes to GEEKSARESEXY [...]

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