Exponential function

  • Thread starter sphyics
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  • #1
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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.
 

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  • #2
Hootenanny
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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.
You answered the question yourself:
f(x) and g(x) are inverse of each other.
 
  • #3
102
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You answered the question yourself:
g{f(x)}= elnx = ???? how to solve this equation algebraically and come to a solution..
 
  • #4
22,089
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What does it mean that f and g are inverses of eachother??
 
  • #5
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g{f(x)}= elnx = ???? how to solve this equation algebraically and come to a solution..
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.

[tex]y = e^{\ln x}[/tex]
[tex]\ln y = \ln e^{\ln x}[/tex]
[tex]\ln y = \ln x[/tex] (by the power rule of exponential functions, since ln e = 1)
[tex]y=x=e^{\ln x}[/tex]
 
Last edited:
  • #6
102
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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.

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

[tex]y=x=e^{\ln x}[/tex]
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:
  • #7
102
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What does it mean that f and g are inverses of eachother??
g{f(x)} = f{g(x)}, and both are one to one, hence inverse of each other.
 
  • #8
192
3
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:

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