Use formula for the SUM of the arithmetic sequence:
(first_number+last_number)/2*(number of members).
So, in your case, it is just:

(1+ln)/2*ln

>Does anyone know a simple formuala to do the following:
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>1 = 1
>2 = 3 (1+2)
>3 = 6 (1+2+3)
>4 = 10 (1+2+3+4)
>5 = 15 (1+2+3+4+5)
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>So if given the number 5, the formula would return the total addition, 15. Thanks in advance for any ideas.
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>Mat