# Homework Help: Finding x

1. Mar 8, 2010

### Toby_Obie

Hello,

Im trying to rearrange to find x from the below (all other values, A, b and c known)

$$A = bx / 1-(1+x)^-c$$

I've rearranged but to no avail, I'm unsure how to isolate x

Any input much appreciated

Thanks

2. Mar 8, 2010

### tiny-tim

Hello Toby!

(you needed to put the index in curly brackets, {-c}, since it had more than one character … alternatively, try using the X2 tag just above the Reply box )

This is the same as (1 + x)-c = 1 - (b/A)x …

I don't think there is a "simple" solution.

But why are you looking for one?

3. Mar 8, 2010

### Toby_Obie

Noted

I've never come up against an equation like this before, just curious whether it can be solved for x ? (new skills)

Is there a general solution to this type of equation ?

Can you point me in the right direction ?

Thanks very much

4. Mar 8, 2010

### tiny-tim

I don't think so.

Real life just isn't that convenient!

5. Mar 8, 2010

### Toby_Obie

Thanks anyway

Anybody else think they know the answer ?

6. Mar 8, 2010

### Mentallic

Really, the problem is very complicated to solve, and for most $c\geq 4$ it's likely impossible to solve!

There is a proof that says for polynomials of degree 5 or higher, there is no way to solve the equation in its general case like quartics and below have been.

7. Mar 9, 2010

### Toby_Obie

Okay

I'm using Excel to approximate x for known values of A, b, c

Thanks anyways

8. Mar 9, 2010

### Toby_Obie

Just a note

Excel Goal Seek function solved my function to 4 decimal places, good tool