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I was looking online for a formula that would allow me to find the sum of arithmetic sequences and I came across the 2 that I have listed below.

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I tried to use both them and they worked. So, I was wondering whether there are some special cases where you should use one formula over the other. Additionally, why does one of the formulas include d while the other doesn't.

What should I use?

Help would be appreciated.

Thank you :)

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    $\begingroup$ It all depends what you're given: since in a A.S. we have $\;a_n=a_1+(n-1)d\;$ , it's easy to see how to get the second formula from thye first one, say $\endgroup$ – Timbuc Apr 22 '15 at 9:19
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    $\begingroup$ if you can find or have all the terms on the right hand side of any of the formula then you can use it $\endgroup$ – Vikram Apr 22 '15 at 9:22

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