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#1 2017-12-03 21:18:00
- Alg Num Theory
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- Registered: 2017-11-24
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Numbers ending in 5
To find the square of a number ending in 5:
Drop the last digit 5.
Multiply the number without the 5 by the number that’s one greater.
Tack on the digits 25 at the end.
For example, to find 85²:
85 “−” 5 → 8.
8 × 9 = 72.
72 “+” 25 → 7225.
∴ 85² = 7225.
It works for numbers with more than two digits as well:
135² = [13×14]25 = 18225.
12345² = [1234×1235]25 = 152399025.
Why does it work? Because for any integer n we have
Last edited by Alg Num Theory (2017-12-03 22:00:27)
Me, or the ugly man, whatever (3,3,6)
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#2 2017-12-03 21:41:50
- ganesh
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- Registered: 2005-06-28
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Re: Numbers ending in 5
Hi Alg Num Theory,
This is an excellent post! Keep it up!
It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.
Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.
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