The question was adding up the numbers 1,2,3,4,5 (your problem is just like 5,4,3,2,1) but my answer included being able to add things like 5,6,7,8,9 (ie the general solution) ... and Milos rounded out the whole discussion.
]]>You are asking how to express Triangular Numbers.
You can do this by using this formula: n(n+1)
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2
or also 0.5n² + 0.5n.
]]>Thanks, w0lfman
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