The capital Greek letter Sigma is used to represent a sum. The number on the bottom of the sigma tells you the lowest value to substitute in for the variable, and the value on top tells you the last value to substitute. Start by plugging in the bottom value to find the first term in your series, then plug in the next integer, and so on until you either have all of your terms listed or enough terms to be confident that you have the r value as well as the first term. From there, you can use the formula for a finite geometric series to evaluate the sum.
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