Question

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

Answer 1

Comment
You are going to have to learn something about f(2)

Step One
f(1) = 160
f(1 + 1) = - 2f(1)
f(2) = -2 * 160
f(2) = -320

Step Two 
The easiest way is just to keep on going. This is recursive which means you use the last answer to get the next one.
f(3) = -2f(2)
f(3) = -2 * -320
f(3) = 640 Two minus' make a plus.

Step Three
Find f(4)
f(4) = - 2 f(3)
f(4) = -2 * 640
f(4) = - 1280  <<<<<<<<< answer

Alternate Method
f(n) = 160 * (-2) ^ (n - 1) 
f(4) = 160 * (-2)^(4 - 1)
f(4) = 160 * (-2)^3
f(4) = 160 * (- 8)
f(4) = -1280