math

If you needed to quickly add 1+2+3+4+5+6+7+8+9 in your head how would you do it? 

No calculators and no need to Google any math tricks or the best way to do it or anything like that. Just curious how others think!

security engineering and applied cryptography saltshaker.blog

match 1 and 9, 2 and 8, 3 and 7, 4 and 6 to get 4x10 and then add 5 = 45

n (n+1) / 2 not that hard to remember if you're aware of it

I think Gauss had it right. 

=(1+9) + (2+8) + (3+7) + (4+6) + 5 
= 10 + 10 + 10 + 10 + 5 
= 45

I’d count on my fingers, run out of fingers, and give up

add first + last (10), multiple by half of the count (4.5) = 45

Group everything to 10 and count full tens and leftover...