Question

Find S25 for the sequence -8, 7, …15n-23

help:3​

Answer 1

For an arithmetic series (note: the sum of the terms in an arithmetic sequence), the formula is:

Sn = ( 2a1 + (n-1)d ) * n/2

Plug in the values of:

n=25

a1 = -4

d = 3

S25 = ( 2*(-4) + (25-1)(3))*(25/2)

S25 = (-8 + 72)*25/2

S25 = (64)(25/2)

S25 = 800