You are not logged in.
- Topics: Active | Unanswered
Pages: 1
#1 2017-12-03 21:18:00
- Alg Num Theory
- Member
- Registered: 2017-11-24
- Posts: 339
- Website
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)
Offline
#2 2017-12-03 21:41:50
- ganesh
- Administrator
- Registered: 2005-06-28
- Posts: 26,430
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.
Offline
Pages: 1