EPTOn February 21 2009 14:36 nevake wrote:
An even more random trick: squaring any number ending in 5
take all the numbers besides the last one (5) and multiply it by the number above it, then add 25 to the end.
ex: 125^2
12 * 13 = 156
125^2 = 15625
Don't know why this particular one came to mind ~_~.
An even more random trick: squaring any number ending in 5
take all the numbers besides the last one (5) and multiply it by the number above it, then add 25 to the end.
ex: 125^2
12 * 13 = 156
125^2 = 15625
Don't know why this particular one came to mind ~_~.
I like to use it often
It's actually very simple to explain. Every number with 5 in the in the end can be presented as 10n+5
So we go:
(10n+5)^2 = 10n*10n+2*5*10n+25 = 10n*(10n+10)+25 = 100*n*(n+1)+25
