# Another inverse problem!

#### sonya

i keep getting stuck on these inverse problems...

the question is

let f(x)=5+2x+5e^x
f^-1(10)=?

i keep messing up on these problems and now im not really sure where exactly 2 start...i know i gotta find the inverse but i would appreciate some help on getting started on this problem.....thx

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#### HallsofIvy

Homework Helper
There is no way to algebraically find the inverse function to f(x)- not with x both in the exponent and as a base.

However, the problem does NOT ask you to find "f-1", it only asks you to find "f-1(10)".

Can you think of a value of x such that f(x)= 10 and what does that have to do with the problem?

#### sonya

well....f(x)=10 when x=0
but i dont c ne relation btwn that and the inverse...

#### HallsofIvy

Homework Helper
Then I think we've discovered WHY you keep "getting stuck on these inverse problems"!! That's the DEFINITION of inverse!

Two functions (call them f and g) are "inverse" to each other (here's the "formal" definition) if and only if f(g(x))= x and g(f(x))= x for all x.

If f(x)= y, then f-1(f(x))= f-1(y)= x.

Since f(0)= 10, f-1(10)= 0.

Whenever f(a)= b, then f-1(b)= a.

if f: x-> y then f-1y-> x

#### sonya

thats it?....how come it looks so simple now?....neways thx 4 clearing things up!...

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