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

I'm trying to find out the equation for how to find out where a number falls in a sequence.

An example sequence would be 3, 9, 18, 30, 45, 63, 84, 108, 135, 165...

108 is the 8th number in that sequence.

2. Relevant equations

If I use the equation (xn+xn^2)/2

(x = 3 & n = 8)

I'll get 108.

What I need is to use the 108 and 3 and get 8.

What's the equation?

Thanks!

3. The attempt at a solution

I dont know :(

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# How to Find what nth-term a Number is in a Sequence

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