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Show that http://www.artofproblemsolving.com/Forum/latexrender/pictures/4cb05ba8a17dcb9e2dcfb9ef1a98966a.gif [Broken] where k is a constant to be found.

I know that a = 2 and that d = 1/2 and substituting these figires into Sn = (2a + (n-1) d) gives n/2 (4 + (n-1)/2) but I just can't see how to get from n/2 (4 + (n-1)/2) to the answer of 1/4n(n+7). I'd really appreciate it if someone could please explain.

Thank you.

Cathy

I know that a = 2 and that d = 1/2 and substituting these figires into Sn = (2a + (n-1) d) gives n/2 (4 + (n-1)/2) but I just can't see how to get from n/2 (4 + (n-1)/2) to the answer of 1/4n(n+7). I'd really appreciate it if someone could please explain.

Thank you.

Cathy

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