How Do You Find the Derivative of an Inverse Function?

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Discussion Overview

The discussion revolves around finding the derivative of an inverse function, specifically focusing on the function f^{-1}(x) at the point x = -0.5. Participants are preparing for a Calculus BC AP test and are analyzing a practice exam question related to this topic.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions their logic in finding the derivative of the inverse function, suggesting that since (1.5, 0.5) is on the original graph, the corresponding point on the inverse graph is (0.5, 1.5), leading to a proposed derivative of -2.
  • Another participant challenges the correctness of the first participant's conclusion, indicating a lack of clarity in their reasoning.
  • A different participant explains the properties of inverse functions, stating that the slope of the original function at a point is the reciprocal of the slope of the inverse function at the corresponding point.
  • This participant asserts that to find the slope of the inverse at (2, 0.5), one must first determine the slope of the original function at (0.5, 2) and calculates it to be 4, thus the slope of the inverse at that point is 1/4.
  • One participant acknowledges a typographical error regarding the value of x, clarifying they meant to refer to x = -0.5 instead of x = 2.
  • Another participant expresses confusion about not finding an x value such that f(x) = -0.5 in the provided table, suggesting a potential misunderstanding of the problem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct approach to finding the derivative of the inverse function, with multiple competing views and some confusion regarding the values involved.

Contextual Notes

There are limitations in the discussion due to missing assumptions about the function f and its properties, as well as unresolved mathematical steps related to the values of x being referenced.

KingNothing
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let me start by saying that I'll be asking a lot of questions here soon as I'm preparing for a Calc BC AP test and I'm using practice exams to help.

On the first practice exam, the first question:
http://img390.imageshack.us/img390/9390/q1graph5bt.png

Find the derivative of [tex]f^{-1}(x)[/tex] at [tex]x=-.5[/tex].
Check my logic:

Since (1.5,.5) is on the original graph, (.5, 1.5) is on the inverse graph. Since the derivative of F at 1.5 is -.5, the answer is the recipricol, -2.
 
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This isn't right. It's not clear why you did what you did to arrive at your answer though.
 
I'm using two properties of inverses:
If (a,b) is on the graph of F, (b,a) is on the graph of its inverse.
The slope of F at (a,b) is recipricol of the slope of its inverse at (b,a)

Therefore, since we want to find the slope of the inverse at (b,a), we need to look at the slope of the original at (a,b).

I know -2 is the correct answer, but since I have just learned the properties tonight, I am not 100% sure that my logic is correct.
 
The question you've asked asks for the derivative of f-1 at x = 2. Nowhere in your work does "2" appear.

If x = 2, f-1(x) = f-1(2) = 0.5, so (2, 0.5) is on the graph of f-1. To find the slope at (2, 0.5), you need to first find the slope of f at (0.5, 2), then take the reciprocal. The slope of f at (0.5, 2) is 4, so the slope of f-1 at (2, 0.5) is 1/4.

You ask for the slope of f-1 at 2, but your work gives its slope at 0.5
 
Sorry, I meant to type at [tex]x=-.5[/tex]. A lot of confusion over nothing!
 
KingNothing said:
Sorry, I meant to type at [tex]x=-.5[/tex]. A lot of confusion over nothing!
I must be missing something! But I didn't find any x value such that f(x) = -.5 in the table! :frown:
Are you sure it's not x = .5? If x = .5, then you seem to be correct. :)
 

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