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HI! I can help you!

Have you seen this sort of sequence before?

Not really

geometric sequences

I don't really understand how to find the sum of a sequence though

so then what's the "magic" formula

okay thank you! in the mean time can you tell me how to find the sum?

(Let's see if I beat the 7 minute video!)

sure!

Okay, I get \[n(\frac{ 2a+(n-1)d }{ 2 })\]

Hmm, 7 minutes. Looks like I pretty much tied with the video :P

so the n would be 9 right? what about the a or d

Yep!

Well, "a" is the first number, so 2 in your case.

And "d" is the difference between the terms, which is -9 (since you're subtracting 9 each time)

Does that make sense at all? :)

yes perfect sense thank you!