Finding the sums of consecutive integers

The Problem Find the sum of the 20 consecutive multiples of 4 that begin with the number 60. 

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The formula for finding the sum of a list of consecutive integers requires that you have the first and last terms in the list. The multiples of 4 are all four units apart. You have an arithmetic sequence with terms the difference of which is 4. (You can find more on arithmetic sequences in Setting the stage for the sums, earlier in this chapter.) So, to find the 20th term in the list of multiples of 4 that start with 60, use the formula an = + d(n - 1), giving you 

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