Question

What is the explicit formula for arithmetic sequence {-20, -5, 10, ...}? Use f(n) where is 1, 2, 3, and so on.

A: f(n) = -20 - 15(n - 1)

B: f(n) = -20 + 15(n - 1)

C: f(n) = 20 - 15n

D: f(n) = 20 + (n - 1)

Answer 1

Answer: B

Work:
Since the general formula of an arithmetic sequence is f(n) = (the first term) + (common difference)*(n-1), you just plug in what was given to you in the problem to that equation. The first term is -20 and the common difference is +15 since the the increase from one term to the next is 15. Doing so produces the equation f(n) = -20 + 15(n-1), which is choice B.