What is the sum of the first 28 terms of this arithmetic sequence? 69, 75, 81, 87, 93, 99, ...

First we find the common difference by subtracting the first term by the second term. 75 - 69 = 6. So the common difference is 6.

now we need to find the 28th term ... we need to do this because we need to use this number in the sum formula.

an = a1 + (n-1) * d

n = term to find = 28

a1 = first term = 69

d = common difference = 6

now we sub

a28 = 69 + (28 - 1) * 6

a28 = 69 + 27 * 6

a28 = 69 + 162

a28 = 231 ... this is the 28th term

now we use the sum formula

sn = (n (a1 + a28) / 2

s28 = (28 (69 + 231) / 2

s28 = (28 (300) / 2

s28 = 8400/2

s28 = 4200 <===the sum of the first 28 terms