# Exponential function

1. Oct 11, 2011

### sphyics

f(x)= lnx ; g(x) = ex

f{g(x)} = ln ex = x; not an issue

g{f(x)}= eln x = ???? (answer for this) f(x) and g(x) are inverse of each other.

how to solve the problem algebraically.

2. Oct 11, 2011

### Hootenanny

Staff Emeritus

3. Oct 11, 2011

### sphyics

g{f(x)}= elnx = ???? how to solve this equation algebraically and come to a solution..

4. Oct 11, 2011

### micromass

What does it mean that f and g are inverses of eachother??

5. Oct 11, 2011

### Number Nine

I suggest trying to prove it for yourself first. If you really can't, I've "spoilered" a proof below. It sounds like you may want to go back and brush up on some of your fundamentals.

$$y = e^{\ln x}$$
$$\ln y = \ln e^{\ln x}$$
$$\ln y = \ln x$$ (by the power rule of exponential functions, since ln e = 1)
$$y=x=e^{\ln x}$$

Last edited: Oct 11, 2011
6. Oct 11, 2011

### sphyics

OMG i was perfect till ln y = ln x; after that i confused myself over if ln y = ln x; does that imply y = x, now i can see the perfect picture, thanks very much for shedding light over the darkness my ignorance :)

Last edited: Oct 11, 2011
7. Oct 11, 2011

### sphyics

g{f(x)} = f{g(x)}, and both are one to one, hence inverse of each other.

8. Oct 13, 2011

### paulfr

Algebraic Solution ??
OK
Let the solution be called "c"
e^(Ln x) = c
Take the Ln of both sides
Ln [ e^(Ln x) ] = Ln c
Use the Power Rule for Logs to get
Ln x Ln e = Ln c
but Ln e = 1 so
Ln x = Ln c
thus
c = x

Last edited: Oct 13, 2011