# Calc pt2

posted by
**Lost One**
.

Original question:

Posted by Lost One on Tuesday, March 5, 2013 at 12:04pm.

Let f(x) = (2/3)x^3 - 2x + 1 with a restricted domain of [1,infinity]. What is the value of (f-1)'(x) when x = 13?

â€¢Calc - Steve, Tuesday, March 5, 2013 at 12:58pm

since x = f^-1(y)

and dx/dy = 1/y'

f' = 2x^2-2

(f^-1)'(13) = 1/f'(13) = 1/(2*169-2) = 1/336

Follow up question:

Okay, I see the logic of this now, but I have another problem. This is for a multichoice worksheet, and the four choices are 1/13, 13, 3, & 1/16. Which one is the correct choice?