The sum of \(n\) terms in an arithmetic progression can be obtained by taking the number of terms being added \(n\), multiplying by the sum of the first and last number in the sum, and then dividing by 2:

\[ \frac{n(a_1 + a_n)}{2}.\]

For example:

\[1+4 +7+10+13 = \frac{5(1 + 13)}{2} = \frac{5 \times 14}{2} = 35.\]

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