Write a program to compute the value of the nth term in the following
series below. Allow the user to choose the value of n to compute. You
should implement the series as a recursive function to get credit. For
a bit of homework extra credit, also implement an iterative
version. Allow the user to decide which version to use.


Write a recursive function to compute the following sum:

a_n = a_{n-1} + 3
a_1 = 5

So a_2 = a_1 + 3 = 5 + 3 = 8


To help you, walk through the individual components:
If n is 5 then:

  5 + 3 + 3 + 3 + 3 = 17