I came across this website showing how to easily compute, in your head, the squares of numbers ending in 5. The formula is:
(n)* (n+1) + 25
Now, the trick is that the 25 isnt "added" it is just the last two digits of the number you find computing the first part of the formula.
Example
1. 85*85
In this case n = 8, therefore
8*9 + 25 = 7225
2. 135*135
In this case n = 13, so
13*14 + 25 = 18225
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2 comments:
Sad in only works for numbers ending in 5.
actually, it works for all numbers, but I couldn't understand the method the guy used for the other squares. Here is the website
http://www.geocities.com/CapeCanaveral/Hangar/8802/
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