# Homework Help: F(11) = 11, f(x + 3) = (11-1) / (11 + 1)

1. Jul 17, 2009

### charbon

1. I'm having some trouble finding what f(x) should be to complete the desired result.

2. If f(11) = 11 and f(x + 3) = (11 - 1) / (11 + 1) , f(2000) = ?

3. The attempt at a solution

f(11) = 11

f(8 + 3) = (f(8) - 1) / (f(8) + 1) = 11

I'm lost from there...

2. Jul 17, 2009

### Staff: Mentor

Multiply the denominator on the left by the term on the right, distribute terms and solve for f(8).

What is the overall problem statement?

3. Jul 17, 2009

### charbon

I tried multiplying both ways with the denominator, it's finding something that works with f(8) that's the problem.

This is the whole problem statement. It comes from an issue of The Mathematics Student Journal where you had to solve for f(1979) but it was changed to f(2000).

4. Jul 18, 2009

### songoku

I'm confused. If f(x + 3) = (11 - 1) / (11 + 1), then f(x+3) = 10/12. It's a constant
from what you've done. i guess it should be f(x+3) = [f(x) - 1] / [f(x) + 1]

If so, you can find f (14), then f(17), then f(20), then f(23), so on..
It will be a repeating form. Try it first ^^

5. Jul 18, 2009

### charbon

Oh oops, sorry about that mistake.

I guess I should have tried looking for a pattern in the beginning. Thanks a lot for your help. :)

6. Jul 19, 2009

### flatmaster

There's something missing here.

f(11) = 11
f(x + 3) = 5/6
f(2000) = ?

You're never told anything at all about the function in general.

7. Jul 20, 2009

### songoku

The information is enough to solve the problem.