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hmm doesn't look like a sequence to me for one

can you post a quick screenshot of the material? so we can see what you mean

Nope, it's a series

hmm ok

n is what you're looking for in this case n = 11

So what's the common difference, look at your previous post what I wrote when you left

So I'll post it again for reference, the common difference is \[t_2-t_1=t_3-t_2\]

I'm really confused right now..

Common difference can be worked out by subtracting the first term from the second

With me so far?

yes!

i'm just writing everything down in my notebook!

Ok cool :)
Well now lets find a, what is a?

is already one of the terms given, or do I have to find it?

a = first term in series

A not already sorry

so its 3

Good!

So we now have d = -2, a = 3, n = 11

sn=11/2 (2(3)+(11+3)-2) correct?

Close, your sign in (n-a) is off but it looks good \[S_{11} = \frac{ 11 }{ 2 }[2(3)+(11-3)(-2)]\]

i got -55 ?

Yup, that looks good

but it isn't one of the choices?

Mhm...

So is your question asking for \[S_{11}\]

yes!

So it's exactly Find S11 for 3 + 1 + (-1) + (-3) +…
right

yes!

Mhm, everything looks right

Ok lets see...a = 3, d = -2, n = 11

Try now :)

So the equation is \[S_n = \frac{ n }{ 2 }[2a+(n-\color{red}1)d]\]

I got 72.8? so -73?

Nope, you must've put in wrong

Use your order of operations :-)