# Integrals and Inverse Functions

1. Feb 25, 2009

### XJellieBX

1. The problem statement, all variables and given/known data
Set f(x)=$$\int^{2x}_{1}$$$$\sqrt{16 + t^{4}}$$dt.
A. Show that f has an inverse.
B. Find ($$f^{-1}$$)'(0).

2. Relevant equations
($$f^{-1}$$)'(x)=1/(f'($$f^{-1}$$(x)))

3. The attempt at a solution
A. f'(x)=$$\sqrt{16 + t^{4}}$$ >0, so f is always increasing, hence one-to-one. By definition of inverse functions, f would have an inverse.

B. ($$f^{-1}$$)'(0)=1/(f'($$f^{-1}$$(0)))

I'm not sure where to go from here for part B. Help?

2. Feb 25, 2009

### ak123456

I think maybe you can find the inverse of the function firstly,then subtitute x by 0 ? I am also focusing on part B

3. Feb 25, 2009

### Hurkyl

Staff Emeritus
(Solving f(x) = 0 should be easy....)

You did forget something, though: to have an inverse, a function must be one-to-one and onto. So, for example, you would need to show that [tex]\lim_{x \rightarrow +\infty} f(x) = +\infty[/itex], and similarly for $-\infty$.

4. Feb 25, 2009

### XJellieBX

Thanks, I think I've got it. I was overlooking some details.

5. Feb 26, 2009

### HallsofIvy

Staff Emeritus
It is not necessary to actually determine f or f-1. Since f is defined by an integral, the "fundamental theorem of calculus" gives you its derivative.