Question

What is the sum of the first 28 terms of this arithmetic sequence?

69, 75, 81, 87, 93, 99, …

Answer 1

first you need to find the common difference by subtracting (consecutive term - previous term)
for example, you can take the first two terms and subtract 75 - 69 to get the common difference

then you need to find the 28th term, which you can find with the formula



once you have the 28th term you can then use the sum formula
S = (n/2)(a1 + an)
where n is the number of terms, a1 is the first term, and an is the nth term (28th term since it wants the sum of the first 28 terms)