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Is there a way to prove that the sum of the arithmetic progression $a_1, a_2, \dots, a_n$ can be calculated by $\displaystyle s_n = \frac{n}{2}(a_1+a_n)$?

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    You should edit your question such that it is clear what you are talking about. Also, mention why you want us to answer the given question, and spend at least a few minutes analysing it yourself, to see if it is complicated enough to request help.2011-06-12
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    To add on Beni's comment, you should know that there are *many many many* types of sequences, and you're referring only a specific type of sequences.2011-06-12
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    ....and more specifically. This identity is true if the sequence is _arithmetic_. It's not true of most sequences.2011-06-12

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