# Question: an arithmetic sum is a sum of the form a...

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An arithmetic sum is a sum of the form a + (a + d) + (a + 2d) + (a + 3d) + ... + (a + nd), where a and d are real numbers and n is a positive integer. The defining characteristic of an arithmetic sum is the fact that the difference of consecutive terms is a fixed constant. Find a formula for the sum of an arithmetic sum that does not involve all of the plus signs.

PLEASE write legibly because sometimes I have to repost questions because I can't read it.