Question

Identify the explicit formula for the sequence given by the following recursive formula:
(see picture below)

A) f(n) = 2 – 4(n – 1)
B) f(n) = –4 + 2(n – 1)
C) f(n) = 4 – 2(n – 1)
D) f(n) = –2 + 4(n – 1)

Answer 1

If f(n) = f(n - 1) + 4, then

f(n - 1) = f(n - 2) + 4

and substituting this into the first equation gives

f(n) = f(n - 2) + 2•4

Similarly,

f(n - 2) = f(n - 3) + 4

so that

f(n) = f(n - 3) + 3•4

and so on.

Note the pattern:

f(n) = f(n - k) + k•4

so that when k = n - 1, we have

f(n) = f(n - (n - 1)) + (n - 1)•4

or

f(n) = f(1) + 4 (n - 1)

so that

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