Problem 738

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

 

Solution:-

 

Find a_n by using the explicit formula for arithmetic sequences:

a_n = a₁ + (n-1)d.

a₁ = -5

a₂₅ = 19

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

S_n = (a₁ + a_n)

175