Solving Limits with L'Hopital's Rule: Differentiating tan^-1(x-pi/4) for x->1

[JFonseka]

Homework Statement

lim
x->1 \frac{tan^{-1} (x - pi/4)}{x -1}

Homework Equations

None that I know of

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

 

[pooface]

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

 

[Feldoh]

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

 

[JFonseka]

Thanks guys.