Question

If f(1) = 160 and f(n + 1) = –2f(n), what is f(4)?

Answer 1

f(4) = f(3+1) = -2f(3) [Using f(n+1) = -2f(n)]

Keep using f(n+1):

-2f(3) = -2f(2+1) = -2[-2f(2)] = 4f(2)

Again:

4f(2) = 4f(1+1) = 4[-2f(1)] = -8f(1) = -8*160

-8*160 = -1280