[Math] Summation Help

[SKG_Scintill]

So my math teacher gave me a book to work with so I can do some math work for later studies I'm aiming for. Now I've encountered something I don't understand, and luck is with me: my math teacher won't be here for the next couple of weeks >.>

Here we go!
I've been given this formula to work out summations:


From what I understand (just so people can see if I interpret the symbols wrong):
A summation equals half of the top number (n), multiplied by (the first solution of the series (a1) + the last solution of the series (an))
With this assumption, I got this question:


Which I worked out like this:


But when I put the question in WolframAlpha (like most of us do I expect), it tells me the solution is 16958.
I don't have pro, so I can't see a step-by-step solution.
What did I do wrong?

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Edit: I solved one summation before where this method DID work. The main difference I see is that it has k=1 instead of k=10, but it isn't explained that it makes a difference.


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Edit minutes later:
Wait, is the first formula incomplete? Because I do get 16958 if I use the length of the series (10 through 70 being 61 numbers), and not n.


Is there something important that's not being told here?