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ex.: 1+2+3+4+5+6+7+8+9+10=55

this it what I mean by "numbers inside "10", including "10" ...I was in bed, thinking of a quick way to calculate that, but with a way bigger number ( ex.: 149581730012375017587312) and I quickly came up with a simple equation : a((a/2)+0.5)=x where "a" is the number and "x" is the sum of all numbers inside it...so, what is the name for that "x"? Since the same thing but with multiplications is a factorial, and this equation is so simple, I assumed there is already a name for it....

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  • $\begingroup$ Funny you saying that $1,2,\ldots ,9$ are in $10$, because they actually are. $\endgroup$ – Git Gud Nov 3 '14 at 17:35
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This is called “the $n$th triangular number”.

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