(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

find the formula for

1,3,6,10,15,21,...

2. Relevant equations

n/a

3. The attempt at a solution

i only can find n>=3

Tn = 3 + [tex]\sum[/tex] i ; i=3 to n

help T_T

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# Homework Help: Find a formula for 1, 3, 6, 10, 15, 21,

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