# Homework Help: Inverse of exponential function

1. Sep 17, 2011

### mark187

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

Give the inverse of this function

N=f(L)=1816-8L

The answer has to be filled in in Maple

2. Relevant equations

3. The attempt at a solution

N=1816-8L

16-8L=ln(N)/ln(18)

-8L=(ln(N)/ln(18))-16

L=-ln(N)/(8*ln(18))+2

Is this correct?
When i fill this in in maple it's incorrect.
I've really no clue why it is wrong.

2. Sep 17, 2011

### ehild

L=-ln(N)/(8*ln(18))+2 is the same function as the original. Choose L the independent variable and N the dependent one. (Just exchange L and N).

ehild

3. Sep 17, 2011

### Mentallic

Yes it's correct. What are you typing into maple exactly? Your syntax is probably slightly off.

edit: Sorry I didn't notice that you forgot to swap your variables.

4. Sep 17, 2011

### mark187

thanks! It's also mentioned that it should be simplified as much as possible.

Does that mean that the 8*LN(18) could be simplified to 8*LN(2*9) and more?

5. Sep 17, 2011

### Mentallic

It's as simple as it can get, unless you believe $\ln{\frac{1}{x}}$ is more simple than $-\ln{x}$ you shouldn't be changing anything.

$\ln{18}$ is definitely more desirable than $\ln{9\cdot2}$

6. Sep 17, 2011

### Ray Vickson

When I let Maple solve the equation for L it gives me exactly what you wrote. Why do you think Maple thinks it is incorrectÉ What type of error message are you receivingÉ

That annoying É is actually a question mark, but when I type it in it gives me that accented E; that only seems to happen when I access this website through Google Chrome!

RGV

7. Sep 17, 2011

### mark187

Well, this assignment was given in a little online test.
The point is, that i can't see why the answer was wrong.
I have 2 chances, so for the second change i will try to swap the variables.