# Limits question

1. Nov 6, 2007

### JFonseka

1. The problem statement, all variables and given/known data
lim
x->1 $$\frac{tan^{-1} (x - pi/4)}{x -1}$$

2. Relevant equations
None that I know of

3. The attempt at a solution
Indeterminate form, so use L'Hopital's rule

Differentiate the top, then differentiate the bottom.
The differential of the denominator is just 1.
However I have no idea how to differentiate the numerator.

Thanks for any help

2. Nov 6, 2007

### pooface

derivative of arctan(u) is: 1/(1+u^2)

Last edited: Nov 6, 2007
3. Nov 6, 2007

### Feldoh

$$\frac{d}{dx}tan^{-1} (x) = \frac {1}{x^2+1}$$

4. Nov 6, 2007

Thanks guys.