Problem 738

Find the sum of the first 25 terms of the arithmetic series: -5, -4, -3, -2, -1, …

 

Solution:-

 

Find an by using the explicit formula for arithmetic sequences:

an = a1 + (n-1)d.

a1 = -5

a25 = 19

The use the formula for the sum of an arithmetic series:

Sn = \frac{n}{2}(a1 + an)

175

 

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