# Homework Help: Discrete mathematics: incursion

1. Oct 16, 2008

### goofyfootsp

1. The problem statement, all variables and given/known data

a 1= 2, a k+1, 2ak-1

2. Relevant equations

What is the 5th term

3. The attempt at a solution

a1= 2
a2=2(2)-1= 3
a3=2(3)-1=5
a4=2(4)-1=7
a5=2(5)-1=9

5th term =9?

2. Oct 16, 2008

### Staff: Mentor

Re: Discrete mathematics: recursion

That would be recursion, not incursion, which has a very different meaning.
The recursion formula you provided is wrong, and I guarantee you that it was not the one you were given in the problem.
From the work you show later on, it appears that you meant this:
a_1 = 2
a_(k+1) = 2*a_(k) -1

So,
a_1 = 2
a_2 = 2*a_1 -1 = 2(2) - 1 = 3
a_3 = 2*a_2 -1 = 2(3) - 1 = 5
So far, so good, but your next terms are wrong. Here's a hint:

a_4 = 2*a_3 - 1 = 2(?) - 1
a_5 = 2*a_4 - 1 = 2(?) - 1

Mark

3. Oct 16, 2008

### goofyfootsp

That equation you presented was correct I did not know how to type it out properly.

I see so

a_4 = 2*a_3 - 1 = 2(?) - 1
a_5 = 2*a_4 - 1 = 2(?) - 1

a_4 = 2*a_3 -1 = 2(5) - 1 = 9

a_5 = 2* a_4-1 = 2(9) -1 = 17

Is that correct?

4. Oct 16, 2008

### Staff: Mentor

Re: Discrete mathematics: REcursion

Yep, it is.

5. Oct 16, 2008